Custom showdown extensions should be linted

ref #4243

- Removing /lib/ from the list of things which aren't linted, as this is
  all custom code
- Fixing up the two files to pass linting, erring on the side of caution
  with disabling regexp rules vs fixing regexes!
Good point, well made.

Should have done the same thing I did with the other code, which is just turn jshint off for a line rather than change the code - much safer. Have done this now.