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 test needs to start with a capital T. Currently it isn't being run by 'go test'.