Add explicit plantuml dependency as optional

[OverkillGuy]

Related to #222 for matplotlib + depending on PR #493. Fixes #482 by adding plantuml as (optional) dependency.

Update docs for new optional deps.

Open questions:

• [ ] What to do about no_plantuml func, see diff
• [ ] What is minimum plantuml version to allow? (currently latest = 0.22)
• [ ] What about the documentation

Note: This PR is meant to be on top of #493, but I couldn't make the PR actually diff from that PR (on my repo) to this HEAD without the PR being on my repo = invisible to yours. So now this PR is from master of upstream, hence contains both matplotlib PR + plantuml PR jumbled together as sequence of commits. Ugh. Feel free to untangle either/both feature proposals as you feel is best, this is just a simple implementation proposal with little actual thought put in, hoping to help your (busy) team.