treeFilter option

[mbostock]

Ref. #1610 (review)

[mbostock]

I just hard-coded this to apply only to the “tidy” tree and cluster layouts (and thereby avoiding it for the indent layout), but we could make this a separate option and use this heuristic as the default.

[Fil]

It's probably nicer (for dev exploration at least) to give function implementations of layouts the same "powers" that named layout have. An alternative if we don't want this to be a user-facing option, is to read a special property (.balancedLabels?) on the layout function. (The two layouts from D3 would be tested like you do here.)

[mbostock]

I’m in favor of an option if you want to make it configurable (and not some alternative mechanism). Perhaps a textBalanced boolean option? Or you could even override the textAnchor option to be mirrored maybe, but perhaps that’s fancier than it needs to be.