On unix, "+ is only supported with xterm_clipboard

If you're on unix, but you don't have xterm_clipboard, you may want to
copy the hash for use within vim (in the unnamed register). Since
xterm_clipboard is irrelevant on non-unix, don't change behavior in that
case.

Fixes #48.
Merge pull request #54 from pydave/patch-1

On unix, "+ is only supported with xterm_clipboard