Quiet flag

I suggest a flag, e.g. `--silent` that silences any output unless circular dependencies, or other non-warning problems, have been found.
```bash
$ madge --circular .
Processed 129 files (3.2s) (151 warnings)

✔ No circular dependency found!

```

Desired usage:
```bash
$ madge --circular --silent .
(nothing)
```
