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`
If I fix the name, the test fails. And it causes another test to fail, presumably because it's altering `HEROKU_GIT_HOST`. (Maybe it should be `HEROKU_GIT_HOST_REGEX ` instead.)

```
--- FAIL: TestAppNameFromGitURL (0.00 seconds)
	git_test.go:33: expected company_name, got 
	git_test.go:39: expected account, got myapp
--- FAIL: TestParseGitRemoteOutput (0.00 seconds)
	git_test.go:65: expected 2 results, got 0
	git_test.go:71: expected remote  not found
	git_test.go:71: expected remote  not found
```