Respect RuboCop file exclusion

[vinistock]

Motivation

Closes #170

RuboCop doesn't respect the exclusion configuration if an excluded path is passed explicitly as an argument (which is what we do in diagnostics and formatting).

Implementation

Pass the --force-exclusion flag to RuboCop, which makes it respect ignored paths even if requested explicitly.

Manual Tests

1. Start the LSP on this branch
2. Open an included file, such as diagnostics.rb
3. Make some edits
4. Verify diagnostics and auto-formatting work
5. Open an excluded file, such as any Ruby file under test/fixtures
6. Make some edits
7. Verify neither diagnostics or formatting are available

[paracycle]

Nice catch!