use TextIOWrapper when communicating with pandoc subprocess

uses 'universal-newline' mode, to avoid confusion with CRLF.

closes #4122
add long heading to nbconvert test notebook

pandoc will wrap it
test that we don't leave any extra CRs in nbconvert
Merge pull request #4205 from minrk/universal-newline-pandoc

use TextIOWrapper when communicating with pandoc subprocess