Add NodeType which replaces the Placer interface (#45)

The `Placer` interface was a weird interface that tried to abstract away nodes being elements or attributes, but it doesn't really make sense. Now `Nodes` just have a `NodeType`.

Render correct HTML5 (#44)

Previously, elements of kind void and empty elements generally would be rendered auto-closing (with a final `/` character in the start tag), which is allowed sometimes but arguably wrong. See https://dev.w3.org/html5/spec-LC/syntax.html#end-tags

This created problems with for example `textarea` and `script`, which cannot be auto-closing, or the browser renders it wrong.

Also clarified in the docs that this library outputs HTML5.

Fixes #42.

Add package docs (#41)

Also update readme with features.

Add Map function (#40)

`Map` makes it easier to build lists of elements without having to iterate.

Render to Writer instead of string (#39)

The Render function has been changed to take a `Writer` instead of returning a string. This makes it possible to generate documents without having the whole content in memory.

This also removes the `gomponents.Write` function, which is now redundant.

Furthermore, the `el.Document` function has been changed to only take one child, as multiple children never make sense for it. (It's not even a child, more a sibling.)

Remove fmt.Sprintf call in attribute Render (#38)

Just concatenating the strings is much faster.

Before:

```
make benchmark
go test -bench=.
goos: darwin
goarch: amd64
pkg: github.com/maragudk/gomponents
BenchmarkAttr/boolean_attributes-8         	 8194791	       139 ns/op
BenchmarkAttr/name-value_attributes-8      	 5143292	       229 ns/op
PASS
ok  	github.com/maragudk/gomponents	2.841s
```

After:

```
make benchmark
go test -bench=.
goos: darwin
goarch: amd64
pkg: github.com/maragudk/gomponents
BenchmarkAttr/boolean_attributes-8         	16755404	        67.0 ns/op
BenchmarkAttr/name-value_attributes-8      	10208625	       116 ns/op
PASS
ok  	github.com/maragudk/gomponents	2.702s
```

Pass attributes as pointers (#37)

Add HTML5 document template (#36)

Add attribute helpers (#35)

Also refactor tests to be table-driven, for readability.

Add element helpers and refactor (#34)

This change adds a lot of element helpers, and refactors:
- helpers into simple, text, and other helpers
- most tests into table-driven tests, so they're easier to read

Thanks to @oderwat for pushing me to improve the tests. 😉

Add contributing instructions (#33)

Add br and hr element helpers (#30)

Remove "fmt" import in readme example (#31)

Also style menu in simple example slightly.

Don't render or error on nil children (#32)

This makes it easier to e.g. conditionally skip a component for rendering.

Add Group function to group Nodes (#29)

Update package doc for gomponents (#28)

Use HTMLEscapeString in Textf (#27)

Add remaining sectioning elements as helpers (#25)

Fixes #22.

Use HTMLEscapeString (#24)

HTMLEscaper is not needed with strings.

Update readme with blog post link (#23)

Group element helpers in different files (#21)

According to the section at https://developer.mozilla.org/en-US/docs/Web/HTML/Element

Add space before auto-closing element (#20)

Add convenience helpers for form elements (#19)

Introduce Placer interface (#18)

When implemented, the `Place` method of the `Placer` interface tells `Render` in `El` where to put a Node. This is relevant for helpers that want to be rendered like attributes, inside the parent element.

Fixes the bug where `attr.Classes` was rendered outside the element.

Add Textf function (#17)

Like Text, but calls fmt.Sprintf before rendering.

Add better example in readme (#16)

Add children varargs to h1-6, b, strong, i, em, img helpers (#15)

This makes it possible to add attributes.

Make attr.Classes a map type (#14)

This makes the usage syntax prettier. Instead of `attr.Classes(map[string]bool{})`, we can just use `attr.Classes{}`.

Add simple example (#13)

Add a test for attr.String (#12)

Add a Write function (#11)

Add a lot of common elements (#10)

Especially add elements that are either used in every document, that nearly always have text content, or that are almost always used with certain attributes.

Don't render twice with children (#9)

Add package el with element helpers (#8)

Add package attr with attribute helpers (#7)

Make NodeFunc and attr implement fmt.Stringer (#6)

Try out codecov.io (#5)

Add Makefile (#4)

Create codeql-analysis.yml (#3)

Add godoc link to readme (#2)

Add package documentation (#1)

Add first implementation of Node, El, Attr, Text