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.
    `["max", "min"]`.
Add examples of `totals_fields`, `totals_formatter` and `tick_formatter`.
bumped to version 1.0.2