Extract source parsing logic to SourceParser

Unexpectedly, the refactoring of `#disabled_lines_in` made about 15% performance improvement. :grin: 

```bash

$ git checkout e5401eb~1
HEAD is now at 1a1d022... Refactor SourceParser#parse
$ for i in {1..10}; do
./bin/rubocop --debug | tail -1
done
Finished in 10.349071 seconds
Finished in 10.33315 seconds
Finished in 10.291683 seconds
Finished in 10.288371 seconds
Finished in 10.2697 seconds
Finished in 10.367297 seconds
Finished in 10.312853 seconds
Finished in 10.310558 seconds
Finished in 10.24348 seconds
Finished in 10.256827 seconds
$ git checkout e5401eb
HEAD is now at e5401eb... Refactor SourceParser#disabled_lines_in and improve performance
$ for i in {1..10}; do
./bin/rubocop --debug | tail -1
done
Finished in 8.943116 seconds
Finished in 9.027017 seconds
Finished in 8.841935 seconds
Finished in 8.814629 seconds
Finished in 8.883739 seconds
Finished in 8.966726 seconds
Finished in 9.073713 seconds
Finished in 8.882651 seconds
Finished in 8.987707 seconds
Finished in 8.929079 seconds
```