[limit_output] updates including allowing per-cell override of limit

[jcb91]

• allow cell-specific override of output length via cell.metadata.limit_output
• make output-limited notes persist through notebook save & reload by adding them as actual output, rather than just calling OutputArea.append_display_data
• limit outputs even over several distinct output events. Previously, stdout outputs separated in time would each be limited separately, rather than taking all of the outputs as a whole.

[jcb91]

(fixes #811)

[juhasch]

Looks good to me.

[jcb91]

there's a long OSX backlog on Travis, so CI checks may take a long time to go through, but since these changes won't be covered by the tests anyway, I don't think we necessarily need to wait for them if it seems to work for people in their own testning

[juhasch]

I think there really is no need to wait for the OSX tests, too.