always pass single lines to transform.push

in InputSplitter.flush_transform

Previous behavior allowed early transforms to confuse later ones
if they handle multiple lines at a time.
only reset the transform once

in flush_transformers, it's not necessary on each segment.

Much simpler, yay!
Merge pull request #6443 from minrk/multiline-input-transform

always pass single lines to transform.push
handle system-wide kernelspecs

in html tets_kernelspecs_api, which can cause counts to be greater than 2
Merge pull request #6445 from minrk/kernelspecs-test

handle system-wide kernelspecs
Add test for prompt numbers
Make sure prompt numbers are actually set
Merge pull request #6449 from jhamrick/execute-preprocessor-prompt-numbers

Correctly set prompt numbers in execute preprocessor