Allow our M-<arrow> bindings to work in the MacVim gui.

It turns out that MacVim sets up some bindings in a default gvimrc file
that collide with the M-<arrow> bindings being setup in our vimrc.
Let's prevent MacVim from setting those bindings by setting
macvim_skip_cmd_opt_movement, and manually setup the ones that don't
collide.