next pass on dropdown API #168

placeholder issue to discuss the next pass of the column_conditonal_dropdown property and friends.

In particular:

• dropdown inside the nested structure should be renamed to options to match dcc.Dropdown
• Similar to the style arguments, it would be nice if all of the dropdown properties had the same prefix, so something along the lines of dropdown_by_row or dropdown_by_condition
• The clearable property inside columns should be dropdown_clearable or perhaps under dropdown_properties.clearable
• Instead of type: 'dropdown', maybe we should have 'type': 'enumerated' and 'cell_renderer': 'dropdown'
• A way to specify per-row dropdown options using row indicies or forthcoming row IDs