Add Change-comment field to cabal edits

[hvr]

In combination with #230 I consider it important we have some additional means to communicate with upstream maintainers beyond informing about the change itself.

Maybe even, writing a change-message ought to be mandatory for any change, as it's a sensitive operation, and each change should ideally be documented as to why it was necessary in the first place.

The comment field should be large enough to accommodate multiple lines (for including e.g. a compiler error motivating the change).