Tons of reactnb features

* Plotting support (based on keywords, or * command prefix)
* Table support (based on ^ command prefix)
* UI support (based on & command prefix)
* Detachable commands (drag a command to detach it)
* Drag to trash (click to un-trash most recently trashed)
* Clear (Ctrl-K) and Clear Including Detached (Ctrl-Shift-K)
* Gather detached from above and left of document boundaries (Ctrl-G)