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:
- Return a non-zero exit code when a module would be changed.
- 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.