Implement attribute plugin

[hagenburger]

New feature

Either an element or a code block should be able to update an HTML attribute.

Specifications

The following combinations should be possible:

• [attribute=value] → attribute="value"
• [attribute="value"] quotation marks must be stripped → attribute="value"
• [attribute='value'] quotation marks must be stripped → attribute="value"
• [attribute='[value]'] quotation marks must be stripped → attribute="[value]"
• [attribute] only the attribute name should be set → attribute

Example usage

``` html [data-xyz=123]
<p>Example</p>
```

# Headline <?: [title="Headline title"] ?>

Help

In case some hints are needed, here is an example which could be checked against.