Mark tags as meta.tag just like HTML bundle does.

This fixes syntax highlighting in some themes, e. g. Eiffel.
Remove rule that I can't find any use-case for.
Improve comment rules.

Here is an example that is now highlighted correctly.

p
  / This line won't get displayed.
    Neither does this line.
    Nor this.
  | This is not a comment anymore.
  /
    This is also a multi-line comment
    Hello world!
p
  /! This will get displayed as html comment.
Include HTML grammar, Slim allows raw HTML.
Match extension-free files.