Add support for rswift.json config to BuildToolPlugin

[TheNightmanCodeth]

I noticed there was no way to set custom arguments to the generation command when using SPM or Xcode BuildToolPlugin

This just allows for configuration via a rswift.json file placed at the project root (or elsewhere if it is added manually to the Xcode project)

The config json would look like:

{
    "generators": ["image", "string"],
    "rswiftignorePath": "/Users/TheNightmanCodeth/project/other/.rswiftignore",
    "additionalArguments": ["--somethingelse", "value"]
}

or any variation (ie. only additionalArguments, only generators and rswiftignorePath, etc.)

Addresses #860 (among others)