Report which files were modified

[sodul]

We want to use unify through the gray project as a pre-commit hook.

It works well but we are getting feedback from our developers that the output is confusing when files are changed:

gray.....................................................................Failed
- hook id: gray
- files were modified by this hook

For example with black more details are provided:

black....................................................................Failed
    - hook id: black
    - files were modified by this hook

reformatted <filename>
...
All done! ✨ 🍰 ✨
15 files reformatted, 1 file left unchanged.

It would be informative to have an output such as:

gray.....................................................................Failed
- hook id: gray
- files were modified by this hook

Unified quotes in <filename>.

This relates to myint/unify#19