Add jupyter-describe function for interactive inspect

[non-Jedi]

jupyter-describe implements an interface (with completion) similar to
emacs builtin describe-function for interacting with jupyter kernels.

Closes #199

This was actually easier than I expected, so kudos for the sane architecture. :)

[non-Jedi]

This should probably be using completing-read. Any idea why it was written this way instead?

[nnicandro]

I copied it over from read--expression called by eval-expression (M-:). My intention with jupyter-read-expression is to parallel read--expression. Also completing-read is more for completing collections right? jupyter-read-expression is supposed to read a line of code for evaluation from the minibuffer while also allowing completion from the kernel within the line.

Although the documentation for jupyter-read-expression does say for completion instead of something more approriate like for evalution so its purpose isn't that clear.

[non-Jedi]

Hm. That makes sense. So for this new purpose I'm putting this code to, using completing-read probably makes more sense, but it doesn't make sense in the context of jupyter-read-expression which is likely to complete multiple symbols. I think this is an acceptable hack though until I finish a completing-read based implementation (I started to do so over at non-Jedi@72b84b8, and it's probably only an hour or so more work on my part to finish, but with 2 young children in the house, I have no idea when I'll have time to finish)?

[non-Jedi]

In either case, I think it probably makes more sense to just refactor jupyter-read-expression to take optional parameters rather than define an internal jupyter--read-with-completion. :/