chore(release): 0.1.21

Inserting nodes from external source made possible by cpunion, NourSa…

…mmour and stephenc222

Fix key behavior in virtualized environment

0.1.18

Silence react@15.5.0 warnings

Add prevTreeIndex, nextTreeIndex to canDrop callback

Fix expand button behavior in forms

Add canDrag API, fix horizontal scrolling

Add canDrop API, fix react@16 warning

Fix hover errors. (Thanks to TooHTooH) Fixes frontend-collective#40