Support conditional derives / custom attributes

[Crzyrndm]

As far as I can tell, #1089 was resolved by the ability to add custom derives. The support for more custom attributes beyond just listing derives wasn't part of the resolution.

I have a case where the derive needs to be conditional (cfg_attr(feature = "feat1", derive(MyDerive))) and the bindings are not being generated from the build script (in this case: to reduce build system dependencies). I have a workaround generating a set of bindings for each combination of conditions but this

• is unwieldy / awkward
• only works for derives and I have at least one case where I need to manually add an attribute currently

It would be appreciated if ParseCallbacks::add_derives could be extended to a more general ParseCallbacks::add_attributes

[pvdrz]

This would also fix #2513

[mkroening]

I have just opened #2866. :)