Support allow non_upper_case_globals lint.

[crlf0710]

I'm trying to create a bitflags with CamelCase naming. However adding #[allow(non_upper_case_globals)] everywhere doesn't suppress the lints.

[binarycrusader]

This is my only disappointment with bitflags; I know that technically the macro isn't generating an enum, but bit flags are frequently defined as enums in C/C++, and it's natural to prefer enum variant casing rules for the bitflags here.

[MagicRB]

When creating ffi bindings to c/c++ libs it gets especially annoying, having to have your bitflag variant named ANY_MODIFIER as opposed to AnyModifier to get rid of those warning is really annoying...

[hexagon-chunt-ricb]

@KodrAus How does #281 fix this issue? The #[allow(non_upper_case_globals)] lint still doesn't work anywhere, the only way to get it to work is to use #![allow(non_upper_case_globals)] for the entire file. Did I miss the solution?