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`
You should use a raw string literal here:
```
`heroku\.com(\..*)`
```

Same goes for the other places with double backslashes.