fix TemplateHelper
improve MetaHeaderCommand

- Don't corrupt files when header can not be detected
- Respect the exclamation character for existing headers
- Inform when skipping a file
fix MetaHeaderCommandTest
Merge pull request #277 from sstok/277-meta-header-add-ignore-or-include-feature-into-gush-yml

Meta header add ignore or include feature into .gush.yml?