store default codemirror mode in only 1 place
do not include specific css in embedable one
catch if cm_config of other key undefined
cleanup Client.close

allows setting `linger` on sockets at shutdown time.
close Client sockets if connection fails

avoids runaway FDs in cases of frequent client failure.
Implement a better check for hidden values for %who etc.
Tweaks to interactiveshell for user_ns_hidden as dict
Add way to install MathJax to a particular profile
Make profile not set a default
Made profile the default install option, install_dir the explicit
simplify join
help_end transformer shouldn't pick up ? in multiline string

Closes gh-4134
ConnectionFileMixin is Configurable

it has `config=True` traits

Currently in master `ipython console -h` will fail because of this.
It doesn't affect 1.0, I'm not sure why.
test that `-h` and `--help-all` work for various IPython entry points

should help catch when we break these things.
fix one remaining Transformer in nbconvertapp

prevented `nbconvert -h` from working
de-decorate help output tests
only test `--help-all`

help-all is a superset of `-h`, so no need to do both,
since these tests are slow.
add missing ProfileDir to nbconvertapp.classes
Reference notebook examples, fixes #4146.
generate choices for `--gui` configurable from real mapping

typo prevented `--gui tk` from working, even though it is supported.
Should prevent similar bugs in the future.

closes #4156

should be backported to 1.1