FakeVim: support :sort command.

When adding include statements to my files, my usual sequence is this:

1. Add the include to the bottom of the includes list
2. Using line-select mode (shift-v), select all includes
3. Sort the lines, using :sort

When trying to do this with FakeVim, I get the error Not an editor command: '<,'>sort.

It would be nice if FakeVim supported the :sort command.