Added -attendee switch to mhical(1).

This allows the user to select which of their attendee addresses they are
responding about, when they have more than one in the invitation.
Added corresponding -a switch to calaccept, etc., functions in replaliases.
Updated mhical to retain DESCRIPTION lines.  Gmail puts a link to the event
DESCRIPTION line along with a message to no edit that section, though it
seems to accept responses if the DESCRIPTION line was removed.
Incremented mhical version number to 0.5.