Add support for generating an apk from a Crosswalk manifest via a --manifest option

[84564221]

Add support for generating an apk from a Crosswalk manifest via a --manifest option

[townxelliot]

Because we didn't get around to implementing it. This project is not being actively worked on right now.

The --app-config option allows you to specify the configuration for the build from a file, if that's what you want. This has a similar effect to using manifest.json, though the field names in the app config JSON file are slightly different from Crosswalk's current manifest.json. See the docs - https://github.com/crosswalk-project/crosswalk-apk-generator#xwalk_apkgen-create-an-apk - for an example.