Add tests for .travis.yml generation, remove options to supply custom .travis.yml command, and refactor TravisYmlView & GenerateTravisYmlFile.
Allow plugins to specify custom travis build commands by creating .yml files and storing them in tests/travis subdirectory. Added example files to ExamplePlugin which are used in generate:travis-yml tests.
Fix small bugs in generate:travis-yml refactoring.
Add extra instructions regarding how to edit travis.yml.twig file.