Option to open tabs on double click instead of single click

Are there any other double-click actions of interest?

Requested by LateNightHacks via user review 2017/10/14.

Note: [looks fairly straightforward](https://github.com/vakata/jstree/blob/master/src/jstree.js#L625)

Start by reading the click-handling code in src/jstree.js around line 625 and identify how TabFern currently opens tabs and handles other double-click actions. Confirm the desired option with the issue discussion, then make tab opening configurable and verify that both single-click and double-click behavior work as selected.

Written by the indexing model from the issue text.