Allow trigger of comments only on additions/deletions

[mjgpy3]

In #106 we added the ability to comment conditionally where a diff contains some substring.

It would be nice to extend this to be able to differentiate on additions and deletions. For example, a config like

UnsafeMentionedInCode:
  paths:
    - "backend/**/*.hs"
  where:
    additions:
      contain:
        - unsafe
  body: |
    :wave: Hi, I see an addition of "unsafe" in Haskell code. Please consider
    finding a safer alternative!

would only comment if the additions to a patch matching backend/**/*.hs contained the substring unsafe. The current additions_or_deletions key will comment if the change is found anywhere in the patch.

[pbrisbin]

I like this and am considering working on it. But I wonder if the configuration DSL could be a little bit better. What about:

where:
  diff:
    contains:  # behavior of current additions_or_deletions.contain
    adds:      # behavior of proposed additions.contain
    removes:   # obvious behavior

Instead of diff, I could also see patch. Not sure which is better.

[mjgpy3]

@pbrisbin I think that reads nicely 👍