Add support for multiple repositories

                      
|Q            |A     |
|---          |---   |
|Bug Fix?     |n     |
|New Feature? |y     |
|BC Breaks?   |Unsure|
|Deprecations?|n     |
|Tests Pass?  |y     |
|Fixed Tickets|#46   |
|License      |MIT   |
|Doc PR       |      |
                      
This is a POC to add support for multiple repositories.

When running `core:configure` command, you can choose which adapter you want to configure, which will add the adapter config to the global .gush.yml file under the 'adapters' key.
This allows you to set the config for E.G bitbucket and github globally.

I added an `init` command that creates a local .gush.yml file in your repository root. In this file, you can set the adapter that the repository should use. If there is no local config file available, then we try and determine the correct adapter to use (this needs some re-thinking) based on the repository remote origin url.

**TODO:**

- [ ] Re-think the determineAdapter method if there is a better way (Maybe using the adapter classes to check for a valid url)
- [ ] Add option to specify custom credentials per repository (not sure how important this really is)
- [ ] Update and add unit tests
- [x] Determine way to switch between github and github enterprise
- [x] Anything else?