cache capabilities and add `#capability?(name)`

[nevans]

The client should keep track of server CAPABILITY responses and response codes. And it should invalidate previously cached values at the appropriate times (e.g. after STARTTLS).

The base spec requires this: (e.g. #32 LOGINDISABLED, #48 AUTH=). And many other extensions require the client to keep track (e.g. #37 LITERAL-). And the parser will also need to be able to "enable" different behavior for some extensions (e.g.
BINARY, UTF8=ACCEPT, IMAPrev2).