Update NEWS for 1.3.0 release

[slager]

• added belated NEWS items for 1.2.0, copied from the current GitHub release
• added NEWS items for 1.3.0 (currently 'development version' in NEWS, but that will change during the release process)
• @kelliemac, can you maybe flesh out NEWS items for things in the Kellie section, and file them into the appropriate sections as needed?
• Feel free to tweak anything else in here as well! I tried to order the items in approximate order of importance for a typical end user who isn't one of the package developers, since that's mostly who the NEWS is for.
• We've been busy! Should we add a note to the CONTRIBUTING.md and/or any other PR guidelines that it would be nice for a NEWS update to come along with each PR, to keep us more current on what we've done? And to try to remind each other/everyone to keep up to date on that with each PR? If we did that now, it would be more visible since it would be in the main branch also.
• Feel free to commit directly to this branch! I'll leave it alone for now.

[kelliemac]

@slager I updated NEWS.md as requested, everything you had there looked great to me!

I also added a PR template that asks about whether NEWS.md has been updated. Personally I don't look at CONTRIBUTING.md on a regular basis so I think this approach is more likely to keep us honest about actually updating the news with each PR. It also asks about unit tests and function documentation - if you think of anything else that should be added, please feel free to edit!