fix history trim backup flag doc
add --keep alias for history trim
fix nbconvert completion

thanks @jakobgager for pointing this out in #4528
add note about the 1000 / --keep variable
bash completion: proper history completion
custom keyboard interrupt handling in  ask_yes_no

Sometimes, Ctrl-C just makes sense as an answer, and re-asking the
question is just frustrating. This changes makes it possible to break
out of the asking loop via Ctrl-C. This is particularly handy for asking
questions from the command line, where one expects Ctrl-c to be a sort
of "Cancel" operation, and is often functionally equivalent to "no"
allow trimming to an empty database (--keep=0)
`ipython history clear` subcommand
bash completion: history clear & trim handle flags
bash completion: fix flags for history w/o subcmd
added note for "what's new" section
another what's new note for history subcommands
sphinx uses double backticks, thanks @takluyver
Specify encoding for reading notebook file.

Closes gh-4562
Specify encoding in remainining instances of io.open
Fixing cell_type in CodeCell constructor.

This was causing the cell_type to be null for CodeCells, which
broke the cell type dropdown in the toolbar.
Merge pull request #4563 from takluyver/i4562

Specify encoding for reading notebook file. and a few other calls to io.open with missing encoding
Merge pull request #4566 from ellisonbg/cell-type-fix

Fixing cell_type in CodeCell constructor.
don't use lazily-evaluated rc.ids in wait_for_idle

iterate through the keys directly,
which should avoid possible KeyError when engine state changes.

should fix occasionally failing tests on Shining Panda
no longer need 3 suffix on travis, tox