Now there's a separate function `_decrypt` which handles `gpg`-decryption.
It is convinient, since both `changes` and `reveal` use it.
Also I have updated manuals to match new features. Also new `brew` repo is now officialy supported.
Added tests for the `changes` command.
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
