Add `message' to erc-match.el highlight types

Add `message' to erc-match.el highlight types

* lisp/erc/erc-match.el (erc-current-nick-highlight-type,
erc-pal-highlight-type, erc-fool-highlight-type,
erc-keyword-highlight-type, erc-dangerous-host-highlight-type): Add
`message' type for highlighting the entire message but not the
sender's nick.
(erc-match-message): Check for the new `message' highlight type and
propertize the message (not including the nick) accordingly.
* etc/NEWS: Announce the addition of the `message' highlight type.