FUTURE: Gather ideas for column strategy

Column strategy should be able to distinguish between existing and new violations on the changed code lines and show new violations only. Current `patchedline` strategy can't understand what exactly was changed in the line (e.g. a whitespace was added between words), so it may show old violations as well.

Investigate ways to implement a column strategy. One of them can be usage of https://github.com/google/diff-match-patch to compare an old and a new line and get the columns within those lines with a new text.