support setting / jumping to notebook-wide marks in Jupyter mode

[edgimar]

It would be great if it were possible to set marks for a particular notebook that would allow jumping to any particular location within the notebook. For example, given a notebook with 100 cells in it, if the 20th cell is currently selected, then when in Jupyter mode, typing ma should store a mark to that location in the notebook. Later, to return to that location, one would type ``a`.

It's not clear what the best way to define 'location' is here, but one way would simply be to associate the location with a particular cell (even if this would be confusing if the cell gets moved to a different part of the notebook).

[lambdalisue]

I agree that it is great but I think it is a bit hard to implement. Currently I don't really have time to discover the way to do, sending me a PR is really welcome ;-)

Ref: #69 (comment)