RFC: Move trailing-whitespace check to a Makefile target

[tkelman]

Also build docs in the same new matrix job, ref #8917 and #2501

Currently this won't run make check-whitespace automatically, but we could add that as well. I'd prefer if we did that, to have it run after the tests are done, at least on Travis. That way the separate matrix job can tell you if there's a style problem (also whether or not the docs build - currently they don't build in pedantic mode, I could switch that off, also doctest won't work without having also built Julia), but you still get the tests running to completion which is valuable information to get even when there are whitespace nits.

Here's an example failing build when there's trailing whitespace, from rebased-away history on this branch: https://travis-ci.org/JuliaLang/julia/builds/45661147

[tkelman]

Another option I was thinking of would be using a separate CI service (one of Travis' competitors) for these little checks, just to make it show up as a separate status item on PR's instead of a larger Travis matrix. I've looked into several, I think the most effective here would be either CircleCI or Shippable. Both would require checking in a new top-level .yml file, and CircleCI requires adding an SSH key to the repository for some deployment stuff that we wouldn't really need.