Create Gameshark plugin [Recommended first issue]

[Baekalfen]

This is a good first PR for someone to get started

This patch should be implemented as a plugin, and maybe fix up what ever it might need to work as a plugin.

We don't have the plugin infrastructure in place yet, so make the implementation in the api directory and expose the functions on the PyBoy object (just like screen, sprite, tilemap, etc.).

Fort-Bonnitar@a8a74b7