allow clearing of a single renderlet or all renderlets

[davisford]

See #776 for rationale

This adds two new functions on chart:

var renderletFn = function (chart) { /* do something with chart */ };
// add a renderlet
chart.renderlet(renderletFn);
// clear it
chart.clearRenderlet(renderletFn);
// clear all renderlets
chart.clearRenderlets();

[r4j4h]

I think gordon's point about the real solution ultimately using d3.dispatch is good to echo here, and I 👍 to having this available in the meantime.

[gordonwoodhull]

#776 covers this in a more general way, merged in beta.2. Thanks for bringing this up, @davisford!