Reconsider jedi 0.19+

[bollwyvl]

Problem

Tab completion is still pretty bare-bones. jedi 0.19 claims to improve the performance, but there is currently no way to control the setting to enable it.

Proposed Solution

Initially, allow re-enabling jedi completion.

If it works well for large packages (e.g. pandas and big DataFrames), enable it by default.

As an additional CLI improvement, it might be possible to pre-cache jedi completions for known packages (#27), removing the "basically always cold cache" problem that this kernel experiences.

Additional context

While jedi and parso are large downloads, the completion experience is strictly better than IPython's built-in one... once it works