Option to disable color output for diff

[yaroslavyaroslav]

The issue is that I wish to forward plandex diff to my tool of choice to review diffs. And I succeeded with that by ksdiff <(plandex diff), but it looks like this there:

�[1mindex 660fa03..d43bc40 100644�[m
�[1m--- a/.github/workflows/doc2dash.yml�[m
�[1m+++ b/.github/workflows/doc2dash.yml�[m

UPD: It'd be a way better to add a general colorless toggle for the whole plandex tool rather than to diff only command.

[danenania]

Thanks @yaroslavyaroslav, agreed! We have a --plain/-p flag for both convo and summary commands, so we should add one for diff as well. A top-level setting is also a good idea, perhaps through an env var.

[ryanwith]

Created a PR that addresses this: #197