Added regexp support for branch specific stages

[corpsee]

Added regexp support for branch specific stages.

See: configuring_project.md and #2.

[corpsee]

TODO: Add documentation.

[corpsee]

Something like that:

test:
  php_cs_fixer:
    allowed_warnings: -1

branch-regex:^feature\-\d$:
  run-option: replace
  test:
    php_cs_fixer:
      allowed_warnings: 0