Allow for more granular documentation parsing

[stuqdog]

Apologies if this issue has already been raised! I did not see it when searching but I might have just not searched hard enough :)

Right now, including documentation when parsing is a simple bool that applies for the crate I am generating for as well as any dependencies I might be including. However, I might want to have short documentation for dependencies, or no documentation for dependencies, or a mix and match where I want documentation for some dependencies but not others. If it was possible to define documentation parsing on a per-dependency basis, that would be much appreciated!