Tagging

With the addition of cell tags to nbformat v4, it's probably a good time to discuss adding a GUI for tagging and filtering cells.  This PR adds a cell toolbar that uses the Bootstrap-Tags library to enable tagging of cells.  It also adds a filter GUI to the main toolbar that allows you to quickly filter cells by tag.

Ping @jasongrout and @sylvaincorlay who requested this, and @ellisonbg who took a look in person and gave me some suggestions.
Why does this toolbar have a unique signature for `register`?