Add an option to disable recursive clone

This change adds the following configuration item to .drone.yml:

    git:
      recursive: false

If the item is left out then the default is `true`, i.e. perform a recursive clone.

I couldn't find where to add the documentation for the new configuration (granted I didn't do an exhaustive search). If you point it out I can update that.

I'm not even sure if you'll be interested in this change, however it was quite useful for me today!
I didn't like this change... But in the spirit of `--depth=0` it seemed appropriate. Let me know if there is a better way!