Also added a test case for this feature.
Other changes:
1. refactored `Makefile` to have `.PHONY` marks
2. added install target to the `Makefile`, now one can run `make install` (with optional `$PREFIX` variable)
3. updated docs for `git-secret.7`, `git-secret-add.1`
4. refactored some commands to `shift` arguments
5. added a test case to the 'other-file-extension' situation
5. added `.github` folder to store templates
Closed#18
