new function to run a single test-file.

Key concepts:
  - no buffer flickering
  - no buffer switching
  - clear after each run
  - support minitest and test-unit
set `default-directory` to project root when running Ruby tests.
`cabbage-ruby-test-ruby-options` to pass ruby options when running tests
Merge pull request #196 from senny/ruby_test_runner

new function to run a single ruby test-file.