allow markdown in heading cells

italics, bold, math, etc.

closes #3053
up default max_msg_size to 1MB
warn about messages dropped due to oversize
style.min.css shoudl always exist...

as IPython and nbconvert shoudl always be in sync now.
Trigger a single event on js app initilized

Usefull to bind to extension loading, to load only once
add PYTHONSTARTUP to startup files

closes #2706
tweak `run -d` message to 'continue execution'

clarify any ambiguity with 'start'.

closes #3464
Added support for markdown in heading cells when they are nbconverted.
Simplify codemirror ipython-mode

make it a subset of python mode, only with configuration
Avoid calling of wrap function in headers of the markdown tpl when is nbconverted.
Added missing space.
We used a better wordwrap function provided by Jinja which let us wrap the titles in the correct way.
Added (to full_html and reveal templates) the possbility to read a custom.css file for tweaking the final html.
Do not wrap headers in markdown.tpl because inconsistency in the interpretation of multiline headers.
Merge pull request #3577 from Carreau/simple-mode

Simplify codemirror ipython-mode

make it a subset of python mode, only with configuration
Merge pull request #3531 from minrk/headingmath

allow markdown in heading cells
Merge pull request #3565 from Carreau/style-min

style.min.css shoudl always exist... don't silently fail.
Merge pull request #3567 from Carreau/app-init-event

Trigger a single event on js app initilized.
Merge pull request #3569 from minrk/pythonstartup

add PYTHONSTARTUP to startup files
Merge pull request #3575 from minrk/run-d

tweak `run -d` message to 'continue execution'

clarify any ambiguity with 'start'.