Arbor.js: new function upstreamArbor(cuts)

Returns a subarbor, consisting of an arbor from the root node
up to (and including) nodes in the cuts map, or end nodes.
WebGLApplication: new skeleton coloring mode "Downstream of tag".

Paints all nodes as grey, except for nodes belonging to subtrees
downstream of a specific tag, specified as a regular expression
(so that multiple tags can be matched).