Rewrite with a better architecture proposed by Fuco1 on #6.

Allow user to customize skeleton-complete with his own skeleton
extracter, matcher, and buffer-filter.
Further refactoring to allow making special matchers.
Fix the matcher movement
Make the buried buffers able to match

In the current buffer of skeleton, matcher will always succeed: the
skeleton is by itself also an expansion. It's easy to remove it.
Update the readme
Add sexp matcher, add interactive functions to expand lines/sexps
Merge pull request #8 from Fuco1/more-matchers

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