Expose a refresh method to be called when HTML is updated

[goldenapples]

As noted in #8, there isn't currently a way to update the contents of the minimap when the HTML of the content it is mapping is updated.

It would be good to introduce a method like $.minimap.refresh() which would do the following steps to refresh the minimap column:

• [ ] detach and remove the existing map column element
• [ ] initialize a new minimap, preserving the settings from the previously initialized map.

[danrubins]

+1