Feature request on martian: MULTIPLE_NOBASE directive storage

I use 'five.grok' in Zope 2, and I have a use case where I would like to have an MULTIPLE_NOBASE storage for a martian directive. It's exactly like an ONCE_NOBASE storage directive, but it can hold more than one value.

The use case is to build a directive 'factory' to define which factories are used to create Zope 2 content. Here we don't want to have the value of the base class inherited, since you want to use only factories you have defined for this class (so they create the correct class).

Patch included.