Add support for Doxygen comments.

[Rapptz]

It'd be nice to have highlighting similar to JavaDoc for Doxygen comments. Example:

Java:

C++:

The syntax for a C++ Doxygen doc comment is just \stuff or @stuff when inside a comment block. It'd be nice to have this type of syntax highlighting. To work with the current syntax highlighting that every colour scheme does, the scope name is:

text.html.javadoc comment.block.documentation.javadoc keyword.other.documentation.custom.javadoc