RFC 2047 Encoder

Offers an encoder for RFC 2047 encoded words.

``` rust use rfc2047::rfc2047_encode;

[test]

fn testencoderfc2047() { asserteq!( "Foo =?utf-8?q?a=C3=A4b?= =?utf-8?q?=C3=A4?= bar", rfc2047_encode("Foo aäb ä bar"), ); } ```

Words are encoded or not encoded as a whole. Only quoted-printable encoding is used.