Limit Chart options to a single parameter

Supporting a variable number of chart options needlessly complicates the
internals of d3.chart.
Simplify `Chart#mixin`

In an effort to honor the single responsibility principle, modify
`Chart#mixin` to operate with previously-created Chart instances.
Additionally, require a unique string name for each mixin.
Implement `Chart#demux`

Provide mixin users with a means to split the input data across mixins.