Add possibility to disable drawer from context

[magrinj]

No description provided.

[rt2zz]

I like this change conceptually, but
a) want to wait until the (hopefully) forthcoming Animated overhaul lands as that may necessitate some larger overall changes
b) am worried that this make use of the disabled prop ambiguous

[magrinj]

I understand your first point, for the second one I don't really think it's a problem :)