Factor out ticker functions and clean up their semantics/usage.

This introduces a new syntax for per-axis properties:

axes: {
  x: {
    valueFormatter: ...
  }
}

The old syntax (xValueFormatter) still works, but is less general.
The valueFormatter and axisLabelFormatter options are now used
consistently between x- and y-axes and have a predictable set of
parameters.

Includes lots of tests for all relevant behaviors.