refactor(dropdown): rename dropdown-item-* vars

The dropdown-item-* vars were inconsistent with their purpose.
As they are used for styling options, let's stick with the dropdown-option-
prefix.
docs(dropdown): demonstrate single/multi selection in the same control
feat(dropdown): multiple selection
style(dropdown.less): remove unused, commented rules
style(dropdowns.less): move around input helper rules
Merge branch 'dropdown-multi'