Added elbow marker to screeplot in plot module

[dokato]

• [x] Does this PR have a descriptive title that could go in our release notes?
• [ ] ~Does this PR add any new dependencies?~
• [x] Does this PR modify any existing APIs?
  • [x] Is the change to the API backwards compatible?
• [ ] ~Have you built the documentation (reference and/or tutorial) and verified the generated documentation is appropriate?~

What does this implement/fix? Briefly explain your changes.

I thought it would be nice to have an option that plots a marker with an estimated elbow at the screeplot.

This simple PR adds it.

[bdpedigo]

i think this just needs auto sorting/formatting run on it

[dokato]

thanks for review @bdpedigo , I think it's sorted now