Make forkmodule-cli work with multiple fork versions

This commit adds the command line option `--fork` to forkmodule-cli,
that lets you define the fork version for which you want to create a
module. It defaults to fork version 3.6

To add versions to support, we use the adapter pattern. Check the
`src/Forkmodule/Version36` directory as an example of how to do that.

usage:

	forkmodule    # This will create a module for version 3.6
	forkmodule create --fork=3.6           # also version 3.6
	forkmodule create --fork=3.7           # also version 3.7