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 
```

url is not always compulsory (https://getcomposer.org/doc/05-repositories.md#package-2)
Even type is not compulsory (https://getcomposer.org/doc/05-repositories.md#disabling-packagist)

Given the content of the attribute `repository` is pasted (as it is) into the composer.json, I wonder if we have to validate this section. We can just let cpmposer do this job