predictable output path for 'goal markdown'.

Instead of outputting to .pants.d/something/... , generate to dist/markdown

(A Twitter team was smooshing Task code into a BUILD file to tweak page generation.
This here change aims to generate .html in a predictable place.
So that Twitter team (or maybe I) can scriptily modify that .html _not_ in a BUILD file, sheesh.)

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/34236236

and added a test

Bugs closed: 535

Reviewed at https://rbcommons.com/s/twitter/r/961/