Add progressive mode

[ssbarnea]

Adds a new option --progressive that ease adoption of the linter as it will only report failure if it detects an increase number of violations since previous commit.

An example of the output when run in progressive mode can be seen below:

WARNING  Total violation(s) reduced from 165 to 0 since previous commit. Will mark result as success
WARNING  Marked 99 previously known violation(s) as ignored due to progressive mode.
WARNING  Listing 99 violation(s) marked as ignored, likely already known

[MarkusTeufelberger]

If I have 5 errors of type A, fix them in one commit but create 4 new errors of type B in that fix, will this fail (since I introduced 4 new errors) or pass (since only 4 errors are now in the repository, even though they are new)?

[ssbarnea]

If I have 5 errors of type A, fix them in one commit but create 4 new errors of type B in that fix, will this fail (since I introduced 4 new errors) or pass (since only 4 errors are now in the repository, even though they are new)?

I rewrote a lot of the implementation and now it will always fail if newer violations are introduced. Take a look and more important, test locally.

[geerlingguy]

This is an interesting feature, for sure—though the first run of Ansible Lint on an existing repo would fail, correct? Or I guess if it runs against a commit that had no new violations compared to the previous commit, then it wouldn't fail.

It would be useful for those who are adopting Ansible Lint on a repo that has a lot of history and needs a lot of cleanup.