Don't check for capabilities in protocol

[filiphanes]

Hi,
I think that user should be responsible to check for capabilities supported by IMAP server and use only them.
Library should not restrict user this way.

1. If capability is not supported, server returns BAD or NO response.
2. With checking in protocol, user could not use this lib with server which supports capability, but doesn't advertise it, for example because it is only partially implemented.
3. It is additional check in every request, protocol should be fast. Of course it isn't significant, but it's a point.
4. It is even more work for correctly checking for advanced capabilities like CONDSTORE, QRESYNC, LIST-STATUS.