Add sexp matcher, add interactive functions to expand lines/sexps

This patch adds the interactive functions to match the lines/sexps. I've also added docstrings to them.

Great work by the way!

However, there's the issue you've been talking about, when the sexp matches something that is multi-line the selection doesn't work (it only picks lines). However, I've found it quite useful for the single-line cases too. However, it would be awesome if we could match multiline regions too. Is this a limitation of ecomplete?