sub: add vader for testing vimscript

Vader seems easier to understand than vspec (probably because I have not
used rspec). Also it is pure vimscript.
Use .md for markdown

Clobbers modula2 -- but I don't use it.
Add F5 for running vader tests
sub: Use my new version of textobj-word-column
Add a vader map for diffing next failure

My solution to junegunn/vader.vim#12. Makes it much easier to see what's
wrong in the failure.
sub: vader diffs expected vs actual

My suggestion was integrated, so my map is now unnecessary.