Support for separate files for header and payload

This version allows you to specify a separate stream to store the Crypt4GH header, rather than prepending it to the output stream. It still defaults to the latter.

Multiple recipients

This release includes a command-line switch to specify multiple recipients

v1.1 - Performance improvements

This version uses the implementation of Chacha20 from libsodium/pynacl (instead OpenSSL).