vimrc: Add command mode mappings for word movement

Never being able to move by word in command mode would always get on my
nerves. In particular if I was trying compose a long command. Sure you
can bring up a command window but that's not what I think of first when
starting to writing a command.

Luckily `cmdline.txt` in vim help has the answer! Since Shift-Left/Right
doesn't work for me and I'm using Control-Left/Right to move between OS
X spaces (not even sure if the Control modifier version works for this
either), create bash/emacs-style mappings to move between words in
command mode.

I'm sorry I didn't find this earlier but it does feel good to have a big
annoyance solved.