Change ctrl-a binding to editor:move-to-first-character-of-line

Considering that the entire Atom core team has ctrl-a bound to move to
the first character and it's also the the default behavior of Sublime,
this seems like a more sensible and useful default option even though
it deviates from text editing norms on OS X.
Merge pull request #3338 from atom/ns-change-ctrl-a-binding

Change ctrl-a binding to editor:move-to-first-character-of-line on Mac