Allow the GH issue + Section as --flags and take news via stdin

[gpshead]

At a high level I wanted this feature:

• Add automation support to blurb add command:
  • New -i --issue option to specify GitHub issue number
  • New -s --section option to specify NEWS section
  • the above include the user friendliness features from Add -i/--issue and -s/--section flags to blurb add #16.
  • New --rst-on-stdin option to read entry content from stdin
• Uses cyclopts for command line parsing instead of rolling our own to reduce our code size, this changes the help format and brings in a dependency.

That led to a rabbit hole, I did the feature in the first commit without cyclopts and really wished we were using it (or the older defopt) given the decorated function pattern being used for commands already.

More code deleted than added while gaining the above features. (until pulling in #16 brought in more, but that makes things nicer anyways)