Custom tags

[samnung]

Allow defining custom tags. It would allow creating something what mixin allows but usable in the middle of the text.

Example of a definition of custom tag:

tag bold_text(klass: nil)
    strong.sans(class: klass)
        - default_block.call

And usage:

p Lorem ipsum <bold_text class="abc">dolor sit amet</bold_text>

Which will produce:

<p>Lorem ipsum <strong class="sans abc">dolor sit amet</strong></p>

This can be done as optional feature and using nokogiri library.