Support named captures in PatternMatching strategy

This MR adds the ability to define named captures in test files and reference them in source files. For example:

mapping:
  # Map API specs to its respective request specs
  - source: '(?<ee>ee/)?lib/api/(?<name>.*)\.rb'
    test:
      - '%{ee}spec/requests/api/%{name}_spec.rb'
      - '%{ee}spec/requests/api/%{name}/%{name}_spec.rb'

and use:

$ bundle exec tff -f tests.yml lib/api/issues.rb lib/api/projects.rb
spec/requests/api/projects_spec.rb
spec/requests/api/issues/issues_spec.rb

⚠ Numbered and named captures cannot be mixed.

Contributes to gitlab-org/gitlab#432986.

Somehow contributes to #2.