Support for removing dimensions.

First cut; needs better tests.

I'm also thinking it might be nice to have `dimension.value(…)` for recomputing a dimension using a new value accessor.  One usage scenario might be filtering multiple ordinal values at once.  Although recomputing a dimension is expensive, it's fairly simple to implement compared with filtering multiple ranges.  Every time a new value is selected, you recompute the dimension using `.value(function(d) { return selectedValues.indexOf(d.field) >= 0; })`.  It should be even simpler than removing a dimension, because you don't need to move any filters around.