Make possible to pass a debug config to the Base.start

Before this patch, the only way to start a debug session was using an environment variable. This way is very intrusive since every thor action will share the same behavior.

So I added a `debug` option to make possible to start the debug session only in one Thor action.

This is a possible fix to https://github.com/bundler/bundler/issues/2790

I'm not sure if I need to update some documentation so please let me know.

cc @indirect @pixeltrix