Add composer repositories configuration

It is a PR for #36.

With this PR, it is possible to use fork, private or local repositories (local maybe not useful). 

```php
<<<CONFIG
repositories:
    - type: vcs
      url: https://github.com/fabpot/Pimple
packages:
    - "pimple/pimple: 1.0.2"
```

In the implementation, I would have preferred to use the CLI composer to add a repository but without composer.json, composer throws an exception.
```
composer config repositories.pimple vcs https://github.com/fabpot/Pimple
[RuntimeException]                               
No composer.json found in the current directory 
```

Given, we are creating a `composer.json` file, what's about creating the whole file and then, runing a simple `composer update`.
/cc @lyrixx ?