Extend catkin_generate_changelog to be able to work with CHANGELOG.md files

[orensbruli]

It would be great to be able to also generate CHANGELOG files formatted in markdown as many repos use this format.
If nobody is against this I could try to make a PR for this.
https://github.com/ros-infrastructure/catkin_pkg/blob/1f28930061c1bb72d25777e3a261b74bdf7d13fa/src/catkin_pkg/changelog_generator.py

[nuclearsandwich]

No objections from me. PR away.

[nuclearsandwich]

Interestingly, I discovered today that the Changelog format is actually specified in an REP https://www.ros.org/reps/rep-0132.html and is explicitly reStructuredText.

I don't think that's a blocker on this work but I am actually going to poll and see if we ought to either create an updated REP that similarly describes a markdown based changelog format or if that's enough weight that projects using markdown changelogs should instead conform to the REP and use rst.

[orensbruli]

Any of both options would be ok for me.

[mxgrey]

Just curious, is there a specific rationale for mandating reStructuredText to the exclusion of markdown, besides the REP declaring "Thou shalt use rst"? Reading the rationale given in the REP, it seems to me that every bullet point could substitute "reStructuredText" with "Markdown" and remain equally valid.

Given that markdown has become a defacto standard on GitHub for simple documents that should be able to display nicely in a web browser while viewing source code, I think it would make a lot of sense to support Markdown.

Also I have the selfish motivation of finding Markdown to be very easy to write while reStructuredText formatting syntax feels bizarre to me.