Box select support

Once there is hover-tool support (issue #91), it would be great if we could support a box-select tool as well, providing essentially the same information as hovering but for a bounding box specified by the user interactively. Adding such support seems relatively straightforward once the hover tool support is in place, but it requires a dynamic query rather than static information provided when the plot is created. Even so, the data can presumably be computed directly from the static information already provided, and so it should not require re-running any of the stages of the datashader pipeline.

Lasso-tool support may make sense as well, again just as a selection of the existing aggregate bins.