Closes itchio#11

- Add only_files query param to /extract
- Test extraction results with only_files

Other details:

- Add Analyzer interface. Originally this was used to handle game and
  music zips differently, but now only GameAnalyzer remains.
- Rename some args and fields for clarity
- Change some structs, namely removing ResourceSpec, which fragmented
  concerns across domains a bit. Logic which affected outgoing headers was
  consolidated under the GameAnalyzer implementation.
- Add optional Metadata field to API result JSON.
- Tests: Replaced assert.NoError() with require.NoError(). The latter stops
  a test immediatly if the assertion fails, which prevents useless and
  noisy panic traces in test output. No dependencies added, already part
  of the testify module.
- Removed unnecessary struct literal type names when they can be inferred,
  per linter.
- Log non-OK notification responses