hi! first off, thanks for maintaining and working on this library. we appreciate it!

i'm implementing a protocol (ActivityPub) and using HTTP Signatures via the Signature HTTP header, but httpsig currently only supports the Authorization header. any chance you could add Signature support?

as a proof of concept, i've done this in snarfed/httpsig@6f0ebfe. example usage in snarfed/bridgy-fed@b123088. it's definitely not the way you'd do it for real, but it does work.

thanks in advance!