Make some more things configurable

*   Stroke color in area renderer can be a function that takes the graph color
    as a d3.rgb color and returns the color of the stroke
*   Format y axis ticks and totals values with a formatting function,
    `ticks_formatter` and `totals_formatter` respectively,
    just like `summary_formatter`
*   Totals fields can be selected by passing an array of strings, e.g.
    `"totals_fields": ["max", "min"]`