enable method
Enables the specified capabilities.
Example: await imapClient.enable(['QRESYNC']);
The ENABLE command is only valid in the authenticated state, before any mailbox is selected.
The server must sipport the ENABLE capability before this call can be used.
Compare https://tools.ietf.org/html/rfc5161 for details.
Implementation
Future<List<Capability>> enable(List<String> capabilities) {
  final cmd = Command('ENABLE ' + capabilities.join(' '));
  final parser = EnableParser(serverInfo);
  return sendCommand<List<Capability>>(cmd, parser);
}