Allow the user to select multiple attribute values for a given attribute

There are occasions where a user may want to select more than one value for an attribute when filtering.

1) They may be open to various options (in which case selecting multiple options allows them to easily compare price and availability).

2) The attribute values may not be fully normalized (in which case they can select all values that may meet their need).

3) The customer may be able to use a range of values (in which case they can select multiple to compare price, availability and features).

We had this functionality on a previous platform and would like to see it on Optimizely.