Support for dynamic header values

[Saichovsky]

I would want to try REST requests with a header that has a value generated from a shell script or some inline language. Something like curl -H "X-custom-header: $SOME_VALUE" https://...

This comes from testing Github hooks, where the X-Hub-Signature has a value that is the hexdigest output of HMAC(sha1). I wished that restclient could calculate this digest each time I sent out a test request.