Let refine accept undefined to remove a selected value

[bobylito]

When implementing a connector like menu it's hard to know what to do to remove a value. The refine is implemented like a toggle but it's not obvious. If we could give a specific value to refine that would help a lot documentation-wise and implementation-wise (no need to store the value).

Impacted connectors:

• menu
• hierarchicalMenu
• refinementList
• hitsPerPage (this one could work but we'd need to document it)
• numericRefinementList
• numericSelector

[Haroenv]

Like React InstantSearch, the refinement value in a refinement list could accept the whole list to apply. This way you can easily select multiple values.