[EPIC] Extensions support

[foxcpp]

Standard extensions

• copy (RFC 3894)
• body (RFC 5173)
• environment (RFC 5183)
• variables (RFC 5229)
• vacation (RFC 5230)
• vacation-seconds (RFC 6131)
• relational (RFC 5231)
• imap4flags (RFC 5232)
• subaddress (RFC 5233)
• spamtest and virustest (RFC 5235)
• date (RFC 5260; Section 4)
• index (RFC 5260; Section 6)
• editheader (RFC 5293)
• reject (RFC 5429; Section 2.2)
• enotify (RFC 5435)
  • mailto method (RFC 5436)
  • xmpp method (RFC 5437)
• ihave (RFC 5463)
• mailbox (RFC 5490; Section 3)
• mboxmetadata and servermetadata (RFC 5490)
• foreverypart (RFC 5703; Section 3)
• mime (RFC 5703; Section 4)
• extracttext (RFC 5703; Section 7)
• include (RFC 6609)
• imapsieve (RFC 6785)
• duplicate (RFC 7352
• regex (draft v08; not latest version)
• ereject (RFC 5429; Section 2.1)
• replace (RFC 5703; Section 5)
• enclose (RFC 5703; Section 6)
• envelope-dsn, envelope-deliverby, redirect-dsn and redirect-deliverby (RFC 6009)
• extlists (RFC 6134)
• convert (RFC 6558)

Obsolete extensions

• imapflags (obsolete draft)
• notify (obsolete draft)

Non-standard extensions

Probably makes sense to be compatible with some of Dovecot's extensions.

• vnd.dovecot.debug
• vnd.dovecot.environment
• vnd.dovecot.execute
• vnd.dovecot.filter
• vnd.dovecot.pipe
• vnd.dovecot.report