keyboard shortcut to add above

[TYMichaelChen]

What is this PR for?

add a keyboard shortcut to add paragraph above to keep difference with previous add paragraph below

What type of PR is it?

Improvement

Todos

Is there a relevant Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-611

How should this be tested?

click into a paragraph and press ctrl+alt+n

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? no
• Is there breaking changes for older versions? no
• Does this needs documentation? no

[Leemoonsoo]

Thanks for the contribution.
How about use shortcut key 'a' for this, like Jupyter/Ipython does?

And I think it's good idea to reserve ctrl-alt-'n' and ctrl-alt-'p' for something related to paragraph focus,
while ctrl-'n', ctrl-'p' moves cursor.

[TYMichaelChen]

@Leemoonsoo : good point. thanks for pointing that out. it's changed to a now.

[Leemoonsoo]

Thanks @MikeTYChen I'm merging it if there're no more discussions