add quiet flag to 'status' subcommand output for checking encryption state

`git-crypt status` detects when a file was staged prior to
the `.gitattributes` file being present, and issues a warning:

```
*** WARNING: staged/committed version is NOT ENCRYPTED! ***
```

Note that `git-crypt` also properly sets return code nonzero in
this case.

Mentioned in #45 was the usefulness of a pre-commit hook
to check proper encryption status. To facilitate such a hook
without needing to stifle stdio, please add a `-q` or other flag
to `git-crypt status`, so it can be used to give a "yes" or "no"
response in hook scripts or similar.