New support for a user level air.toml. This is used as a fallback instead of Air's default settings whenever there isn't a project level air.toml available (#309):
On Linux and macOS, place it at ~/.config/air/air.toml.
On Windows, place it at %APPDATA%\air\air.toml.
For shared projects, we highly recommend using a version controlled project level air.toml instead to ensure that everyone shares the same settings.
