avoid illegible ticks when the size of the domain exceeds x times the number of ticks

[Fil]

http://localhost:8008/?test=mobyDickFaceted has approximately 8 times the number of autoticks (26 letters vs 3.3 autoticks), and it's acceptable because it's single letters (there's even a bit of space left).

With a threshold at 10, and setting the height at the ultimate point before this kicks in, it's really become illegible.

With a threshold at 8, it's just barely begun to be illegible.

With a threshold at 5, it's packed, but still legible.

closes #74

[mbostock]

My preference would be to do this at the tickFormat level so that the tick lines are still drawn but only some of them are labeled. For example like so:

tickFormat: ((f, i) => d => i(d) < d ? null : f(d))(d3.utcFormat("%-m/%-d"), d3.utcWeek.every(4))

[mbostock]

Also related to #212, where I’d like to special-case the behavior for ordinal date domains if possible. Though maybe it’s not possible since the dates could be irregularly spaced, so I’m not sure how we’ll know which ticks to label.

[Fil]

needs an update with the new tickSpacing option