Add possibility to specify the path of an auth.json

It would be nice to have the option to specify the location of the `auth.json`.

In our case we have a repository with a `./composer.json` that provides tools to build and deploy the application and a `src/composer.json` that installs the dependencies of the actual software which has its code source in `src/`.

We need to install modules that require authentication in both `./` and `src/` and therefor it would be awesome to have the possibility to use the `src/auth.json` in the root `./composer.json` as well.

Maybe it could be useful for some users to specify multiple paths. We should still respect the user's `{home}/auth.json` and the `auth.json` at the same path as the currently evaluated `composer.json`.

A config example could be:
```json
{
"config": {
"auth-files": [
"src/auth.json",
"../auth.json",
"vendor/my/package/auth.json"
]
}
}
```