Allow other providers

Saw this tweet: https://twitter.com/emning/status/514185237002088448, thought I'd add it in! (Would be useful for me too because I host some modules in private Github Enterprise or GitLab instances)

After this change, you can add the following to `modulesync.yml`
```yaml
---
namespace: awesome-modules
git_user: gitlab
git_provider_address: gitlabserver.cool.dev
```

Then you'll get this:
No config file found! Using defaults
```
Syncing mysql
Cloning repository fresh
Cloning from gitlab@gitlabserver.cool.dev:awesome-modules/mysql.git
```

I should probably update the docs too, and maybe add some basic acceptance tests for the change?