Stop indenting empty lines when expanding snippets
Remove unused ProcessFile function
Add extends keyword support in .snippets files

The idea comes from UltiSnips. With "extends foo" in a .snippets file,
snippets for the foo filetype are also enabled.

This will replace scope aliases. Additionally it should increase
compatibility between SnipMate and UltiSnips.
Deprecate scope_aliases

The extends keyword in .snippets files replaces them.