Add revisioning option to allow for archival message fidelity

When you change the message type at one point all the already existing messages are also affected.

For example you make a message type node_created:
A node with title %title was created.

Then later you replace this with:
A node with title %title was created by %author.

All the old messages will also display "...by %author". This could be solved by using versioning on the message text and link the message table by this version id. Because if it isn't done that way the only solution is to create a new message type everytime you modify the message type.

Since I can imagine the need to change a message text sooner or later is pretty common I wonder if this module is actually useful without that feature?