Handle value uses publish execution result

[romainfrancois]

instead of display() which has to be called explicitly

[romainfrancois]

This can be followed up, but the idea of the pull request is that only the last expression is being auto-printed, and goes through publish_execution_result() so that this is aligned with the test strategy we inherited from the 🍪.

• This departs from IRkernel is doing
• This has a workaround for ggplot because the plotting only gets triggered by the print()ing so we have to force print them
• This also means that more fancy bundles have to be explicitly asked for, with display()

closes #33

[romainfrancois]

This needs some follow up to handle html widgets, but I'll open a new PR