Allow printing out only reformatted filenames

[kitbellew]

Description

Rather than printing all filenames that were processed, add an option to display only those which were modified.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Ran locally. Also, modified unit tests.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[kitbellew]

@SimonJPegg PTAL