Custom marking of encrypted messages

Added mark_encrypted which works like mark_encrypted in fish-irssi. Depending
on mark_position either prefix or suffix message with string found in
mark_encrypted.
bump version
fixed issue were 'set ' was prepended to key when not specifying a target
allow key removal without specifying a target
prevent unallowed values from being set

limits mark_position to be only set as 'off', 'begin' or 'end'