"use strict" in most (if not all) our javascript

+ fix one variable leaking in global nameespace
rename call methods to transform and postprocess

- The `call` methods for nbconvert transformers has been renamed to
  `transform`.
- The `call` methods of nbconvert post-processsors have been renamed to
  `postprocess`.

  pinging @Carreau and @jdrefer, who probably knows who'd be affected by
  this change...  maybe @damianavila, @jakevdp
replace 'transformer' with 'preprocessor'
fixup minor typos from search/replace
renamed transformer to preprocessors
minor typo
capture rich output as well as stdout/err in capture_output

closes #3742
add outputs property on CapturedIO
add --no-display flag to %%capture

document this new magic in whatsnew, and added docs to the resulting object.
updated Basic Output notebook with  new %%capture
default values for RichOutput attributes
tests for utils.capture
subclass RichOutput in ExecuteReply

they are very similar

add another test for good measure.
Merge pull request #3969 from Carreau/usestrict

"use strict" in most (if not all) our javascript

plus fix one variable leaking in global namespace
Merge pull request #3744 from minrk/capturedisplay

capture rich output as well as stdout/err in capture_output
minor bug fix, define 'var' in dialog.js.

The 'cell' variable isn't defined and causes and error in the
console
Merge pull request #4044 from ivanov/transform-not-call

rename call methods to transform and postprocess
Merge pull request #4068 from Zsailer/jsfix

minor bug fix, define 'cell' in dialog.js.
rename post_processors submodule to postprocessors
Update supported Python versions in tools/test_pr