Make writer & post-processor aliases case insensitive.
Simplify
Revert "Simplify"

This reverts commit fdac8f30fd825e3f359e7cdfc6463829042699da.
Merge pull request #3947 from jdfreder/case_insens

nbconvert: Make writer & post-processor aliases case insensitive.

allows `--post pdf`, among other things.