Add the ability to hide members in Hierarchy Slice

[lav1]

Add the ability to hide members in Hierarchy Slicer, as it would provide a better visual for ragged hierarchies.

[ggalloway]

The custom visual has a Hide Members setting that allows me to hide blanks and ragged hierarchies look right. The out-of-the-box slicer is unusable with ragged hierarchies. All sorts of "(Blank)" members show up.