Allow guesses to contain file name patterns

This enables mapping like:

mapping:
  - source: '(ee/)?app/models/(.+)\.rb'
    test: '%sspec/models/%s/**/*_spec.rb'

Map app/models/alert_management.rb to:

spec/models/alert_management/alert_assignee_spec.rb
spec/models/alert_management/alert_spec.rb
spec/models/alert_management/alert_user_mention_spec.rb
spec/models/alert_management/http_integration_spec.rb
spec/models/alert_management/metric_image_spec.rb

Closes #25 (closed)