Allow additional headers in JWE encryption (Note: Addtional header are already supoprted in JWS signing)

[debanshuk]

JWS and JWE, both spec have the provision of custom (i.e. user-defined) headers. python-jose supports passing custom headers in the headers parameter in jose.jws.sign() method. But the same is not supported in jose.jwe.encrypt() method. This issue aims add this support.