API to plug-in extra parsers

[gossi]

In a composer.json the extra node is reserved for third-party applications to put their stuff in. Third-party devs should be able to plug-in their parsers into composer to provide a nice API on their own, instead of the JsonObject and JsonArray types.

The API should be able to provide two endpoints.

1. registering the parser:

ComposerPackage.addExtraParser("node", MyDomain.class);

1. the retrieving:

ComposerPackage composer = new ComposerPackage(new File("composer.json"));
MyDomain domain = (MyDomain)composer.getExtra().getAsObject("node");