Add support for setting indicator color (#29)

* Add support to assign a color to a slicer, and show indicators in that color

* Update example

* use traces instead of shapes to avoid overwriting layout.shapes

* only draw at corners

* Nicer default color?

* Replace 'index' and 'pos' stores with 'state' store

* Use D3 colormap, and tweak appearance of current-slice color hint

* Rename variables in (internal) info object, for clarity

* slicer indicators show visible range too

* prevent unnecesary high res pulls

* Show slicer views in 3D view, in one example

* tweaks

* fix example

* limit state.xrange and state.yrange to the volume bounds

* prevent camera reset

* use actual plot size

Author: almarklein