support `%matplotlib qt5`

just needed the key, since `%gui qt5` already works
support %matplotlib nbagg

we don't need to do anything, just add keys to a couple of dictionaries
Merge pull request #6587 from minrk/mpl-qt5

support `%matplotlib qt5` and `%matplotlib nbagg`
Fix out of date docstring
use standard html
Merge pull request #6592 from Carreau/html

use standard html
note how to start the qtconsole

at the top of the qtconsole doc
Merge pull request #6593 from minrk/command-doc

note how to start the qtconsole
remove nbformat.v4 spec files

nbformat 4 won't be in IPython 3.0
msgspec 5: stream.data -> stream.text
Removing useless help text from "-i" command line option.
Merge pull request #6597 from ngoldbaum/master

Removing useless help text from "-i" command line option.
Merge pull request #6488 from jhamrick/lock-cells

Proof of concept for preventing cell deletion via metadata
Use codemirror for error messages
Shell command formatting.

Makes the command to start the qtconsole consistent with the rest of the
shell commands in this document.
Clarify escaping for $.

Repeat that $$ is used when a literal $ is desired.
Merge pull request #6600 from vaibhavsagar/qtconsole

Shell command formatting.
Merge pull request #6558 from jhamrick/kernel-start-failure

Handle kernel start failures more gracefully
Merge pull request #6582 from jdfreder/symmetric_state

Symmetric widget get/set_state
Surround $ with backticks