VIM-439: Implement :sort command.

- Supports "n", "i" and "!" arguments.
Merge remote-tracking branch 'selesse/vim-439'

Conflicts:
	src/com/maddyhome/idea/vim/ex/CommandParser.java
New IdeaVim API compatibility
Added |:sort| to index
Added Alex Selesse to the list of contributors
Fixed nullity warnings
Immutable local variables where possible
Local class name conventions
Nullity and immutability
Updated CHANGES