Macaulay2-jupyter-kernel

- Bug fixes

- Clean up asset installation - codemirror and kernel spec.
- Add custom shortcuts.
- Add custom help link.

- Guard against echo in pexpect.
- Added `9,0` as a kernel-restart-and-clear keyboard shortcut.
- Added `debug` interpreter option.
- Added `original` display mode, returning the same output as M2's interactive session,
making it easy to copy output in text-only context.
- Other small tweaks.

- Rewrote the REPL part:
* It no longer uses regexes (except for inferring the input \).
This had a tremendous effect on stability and speed.
* Added finer timeout control.
Now if a statement timeouts, then whole cell execution is interrupted,
and this is clearly reported to the client.
* Macaulay2 exceptions and stdout are always printed to the cell's stdout.
I removed stderr formatting, since it did not fit nicely with the rest.
* Decoupled the REPL from the kernel-proper class.
It can now be used directly in Python3. Will add demo later.

- Put codemirror mode in the correct place.
This fixes GET 404 error that prevented syntax highlighting when running remotely.
This isn't documented at all but the fix works fine.
- Add comment in Macaulay2 using Cmd-/ or Ctrl-/.

- Add the nbextension to the distribution.