Add option to specify git host as regex

With the old heroku command line it was possible to set `HEROKU_GIT_HOST` to a regular expression, which was very useful when using Heroku with multiple accounts (http://git.io/7b7oug).
This adds this functionality using a new environment variable: `HEROKU_GIT_HOST_REGEX`
This is questionable. It misses escaping several other regexp metachars. See http://godoc.org/regexp/syntax.

It would be better to treat the literal hostname case separately, using the same logic that was here before this patch.