APPEND support is incomplete

[jonhoo]

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]) { /* ... */ } 

See the original issue here: mattnenterprise/rust-imap#60

[cyphar]

mattnenterprise/rust-imap#61 was my PoC for this.

[mblarsen]

Would love to see the date feature implemented. Should I try to revise the old PR?

Any thoughts on the api? Keeping append as is and add append_with_flags_and_date to avoid breaking anything?