Add a selectable option to nodes so that some can be selected and others can serve as expansion click targets.
Add singleClickToggle feature to allow node expansion/contaction on click. Eases mobile interaction.
Add emptyIcon config to allow changing the expand/collapse icon placeholder for nodes with no children.
Merge branch 'single-click-toggle'
Merge branch 'selectable-node-config'

Conflicts:
	README.md
	src/js/bootstrap-treeview.js
	tests/tests.js