Use label_data from mexisme/gmail-imap

Much cleaner way of parsing label strings using IMAP's built-in ASTRING
parser.
Enable negative X-GM-ID support

Gmail can return negative values for X-GM-ID's, which don't conform
to Net::IMAP::ResponseParser's regexp pattern.

This commit modifies the regexp to look for a possible negative value. No
conflicts have been identified, though testing has not been extensive.