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`
Reading the code for appNameFromGitURL I would expect this to return `"myapp"`, not `"company_name"`. I suspect the test is just wrong, but it could be the function being tested.