Add reporter for format summary

[paul-dingemans]

Description

Add reporter for a format summary of one line per file. This reporter support colored output in same way as the plain reporter. Like other reporters it allows it content to be written to a separate file. The reported proceduces output when running in format and lint only mode. See examples below.

With lint only:

$ ktlint-dev --experimental --reporter=format --relative --color 
Foo3.kt: Format required (2 violations need manual fixing)
Foo2.kt: Format required (1 violation needs manual fixing)
Foo1.kt: Format not needed (no violations found)
Foo4.kt: Format required (all violations can be autocorrected)

With format:

$ ktlint-dev --experimental --reporter=format --relative --color -F
Foo3.kt: Format not completed (2 violations need manual fixing)
Foo2.kt: Format not completed (1 violation needs manual fixing)
Foo1.kt: Format not needed (no violations found)
Foo4.kt: Format completed (all violations have been fixed)

Closes #821

Checklist

• PR description added
• tests are added
• CHANGELOG.md is updated

In case of adding a new rule:

• README.md is updated
• Rule has been applied on Ktlint itself and violations are fixed