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.
Allow .lvimrc files to override fswitch values.

Localvimrc fires at BufNewFile and BufRead time, which means the current
fswitch autocommands come afterwards and can clobber values set in the
.lvimrc file.  Let's check for the existence of the variables before
setting them, allowing .lvimrc files to override our defaults.
Merge branch 'allow-m-arrow-mappings-in-macvim' of https://github.com/jszakmeister/vimfiles