Formatting plugin improvements

Allow modifying formatter args

This approach is a lot more flexible. The user can supply arguments which are then prepended to our own args.

One can use this to run a formatter from wsl for e.g., by doing

{ "path": "wsl", "args": ["clang-format", "--fallback-style=Webkit"] }

Allow overriding formatter config from .kateproject file

The user can use it change settings for a particular project.