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 variable should be called `s`.

https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Variable_Names
http://research.swtch.com/names