APPEND support is incomplete

[cyphar]

At the moment APPEND only supports setting the body of the message. However, RFC3501 allows users to specify the flag set to be applied to a message, as well as the date the message was received. Quoting the specification:

6.3.11. APPEND Command

   Arguments:  mailbox name
               OPTIONAL flag parenthesized list
               OPTIONAL date/time string
               message literal

We could change the append method to:

use chrono::DateTime;
use chrono::offset::Utc;

fn append(&mut self, folder: &str,
          flags: Vec<String>,
          datetime: DateTime<Utc>,
          message: &[u8]) { /* ... */ } 

[jonhoo]

Let's move further discussion to jonhoo/rust-imap#60.