Add a `validate` mode

Primarily intended for CI usage. There should be a nice way to know whether purty would change the module without actually changing the module.

There are probably two parts to this being complete:

1. Return a non-zero exit code when a module would be changed.
2. Show a diff of what would change.

Neither one really seems like it should impede the other, so we might want to split this issue when working on it.