core/keybindings: Insert a space after a comma, automatically

Borrowed the idea from a blog post[1], tried it, liked it, enabled it.

 [1]: http://ericscrosson.wordpress.com/2013/04/05/minimizing-keystrokes-required-by-punctuation/

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>