[command] Implement SPARQL document parser and add support for #INCLUDEs

First version of the new 'Execute SPARQL Query' command that now offers
support for section metadata and ... ratatatam ... #INCLUDEs. Yay!

By using Includes, one can reuse named sections all over the place.
This way, it is possible to write typically used snippets once, and
then just "call" them from anywhere else when needed. This is
especially helpful for SPARQL beginners or when iteratively developing
complex queries where one wants to see the effect of a previous query
immediately.

When executed, every included snippet is listed separately in the web
preview window of TextMate. That way it is easy to track down issues
with intermediate states (and not only after all queries just have
finished).
[snippet] Add some SPARQL snippets
[cosmetics] Add SCSS document for web preview of "Execute SPARQL Query"