Vim: add some more feature detection

This is by no means all the things that could be protected by feature
detection checks, but it is a few things that stood out in an initial
pass.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Vim: set up spell checking for Git commit messages

"Good" words can be added to a machine-local spellfile (dictionary) with
`zg`, and "bad" words can be struck out from the spellfile with `zw`.

For more details, see `:h spell`.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Vim: use bracketed paste mode

This works on my (OS X) laptop inside and outside of tmux.

This is somewhat black-magical, but pleasingly works even in edge-casey
scenarios like alternating pastes between Vim and the shell (in
different tmux panes; or by foregrounding and backgrounding the editor
process).

On CentOS, looks like it only works outside tmux. This might be because
the CentOS box I'm using is using tmux 1.6 while my the Mac has 1.7, so
I'll be trying upgrading if I can solve the build issues I had when
trying to update to 1.7 previously.

The tmux 1.7 changelog does report that "bracketed paste mode" support
was added:

  http://sourceforge.net/projects/tmux/files/tmux/tmux-1.7/

but the other references I found refer to the addition of a  `-b` option
to the tmux `paste-buffer` command, which isn't at all relevant here:

  http://www.mail-archive.com/tmux-cvs@lists.sourceforge.net/msg00572.html

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Vim: add comment label to Clipper-related config

Signed-off-by: Wincent Colaiuta <win@wincent.com>