Provide mechanism for golden tests

In order to easier maintain the project we should consider providing golden test mechanism.

Proposal
1. there would be dedicated `golden` folder in tests
2. in that folder for each feature you are trying to test you would provide:
- `featurename-input.hs`
- `featurename-output.hs`
- `featurename-config.yaml`
3. The mechanism would run `stylish-haskell` against provided input file and configuration and compare . results with output file

Benefits:
1. This should simplify testing process (matter of adding yet another golden test)
2. We could expect community to attach golden files whenever they are requesting a feature or informing about a bug ("this is the source that i have, this is the output i would like to see"). Issue than would not be considered resolved until the golden test provided is not added to `golden` folder and that test is not passing