Custom Types (root-only)

This pull request allows a user to define custom package type locations in the root `composer.json` file. This happens seamlessly by simply defining [Custom Install Paths](https://github.com/composer/installers#custom-install-paths) by `type` For a complete description, please see #180.

Ideally, this should also support names as well, but since [Composer\Installer\InstallerInterface::supports()](https://github.com/composer/composer/blob/master/src/Composer/Installer/InstallerInterface.php#L32) only provides $type as a string, I don't think that is possible right now.