[WIP] Improves source map generating (map more tokens)

This PR adds more mappings to the source map.

It seems reasonable to map all kind of tokens back to the original source!

Will create more sophisticated unit tests for [`perl-libsass`] [1] (using [`OCBNET::SourceMap`] [2]).
Already started to document my findings about source map internals in a [wiki page] [3]

[1]: https://github.com/sass/perl-libsass
[2]: https://github.com/mgreter/OCBNET-SourceMap/tree/POC
[3]: https://github.com/sass/libsass/wiki/Source-Map-Internals