CB-5416 - Need to auto generate manifest.webapp with appropriate plugin permissions

Please see discussion on [this other pull request](https://github.com/mozilla-cordova/cordova-firefoxos/pull/2).

The config parser job is to update the manifest, I've added permissions and a couple other properties that were already available in `config.xml`.

I also changed this to not modify the manifest if a permission with the same name already exists. This enables manually changing permissions in the manifest. The down side is that the permission will stay even if the plugin is removed - it needs to be removed manually.

Using master for lack of better branch. ffos is really old and I can't request a new branch to be created when doing a PR :(