lint: repo-wide super-linter

[thermokarst]

this replaces the existing python-only linting with github's "super-linter":

https://github.com/github/super-linter/

this linter checks all sorts of things, in particular markdown! i added a rule for enforcing line length (this really helps with diffing prose). i set it to 80 cols, just because that's already a fairly common line length in our other projects, but if we want to bump that up to something like 100 or 120, just say the word! this also does some light spellchecking and grammar checking, which is neat.

[thermokarst]

also, check out this sweet mascot: