Add smart "Home" for mac users. Fixes #2432

[soswow]

Sorry that I didn't make separate pull-request for different bugs (now I got smarter)
But this case was discussed in the issue #2432 itself
and in my previous bulk pull-request: #2435

Just in case @peterflynn 's comment will gone when I made changes to the original pull-request:

I wonder about this... it seems like most Mac editors don't have this behavior. In Coda, TextMate, and > Espresso, Cmd+left always goes to column 1 regardless of whether there's leading whitespace;
instead, Fn+left has the "smart home" behavior (Fn+left also does "smart home" in Brackets already).
But to throw a wrench in that, Sublime differs: Cmd+left means "smart home" and Fn+left means "go to
start of document" (more like non-code-editor Mac apps).

It might be worth separating this out into its own pull request so we can discuss it separately and
neither one will hold up the other one from landing...

[peterflynn]

Curious if @njx has any thoughts on this. Glenn didn't seem to have strong feelings about which behavior is correct, and I'm not much of a Mac guy so I don't either...

[soswow]

Seems like @njx already shared his thought in the issue itself

Seems like reasonable behavior for Home--Sublime does this on the left, but not on the right; Cmd-right arrow always goes to the very end of the line even if it ends with whitespace. One reason not to do it on the right is that you might want to be aware if there's trailing whitespace on the line.

[jasonsanjose]

Looks good on mac and windows. Merging.