Allow a `:dirs` option to `SourceAnnotationExtractor.enumerate`

Currently, you can only _add_ other directories to the defaults using `ENV['SOURCE_ANNOTATION_DIRECTORIES']` with `rake notes` or `rake notes:custom`.


This allows a custom rake task to explicitly set the directories using:

```ruby
SourceAnnotationExtractor.enumerate 'TODO|FIXME', dirs: %w(app lib), tag: true
```