Document the class mappings features

Signed-off-by: martin f. krafft <madduck@madduck.net>
Handle nonexistent nodes if class_mappings matches

If a class_mapping matches, then there is no real need for the storage
backend to actually find a node record, so if it raises NodeNotFound, we
can gracefully catch this.

This could probably be made configurable, but it's do for now.

Signed-off-by: martin f. krafft <madduck@madduck.net>
Disable implicit node creation

With mappings, the strict requirement for nodes to be defined, e.g.
with yaml_fs through existence of a file, is theoretically no longer
required to be strict.

Let's keep it for now, though, but export the behaviour to the function
signature of nodeinfo()

Signed-off-by: martin f. krafft <madduck@madduck.net>
Change to using a list of mappings to maintain order

Signed-off-by: martin f. krafft <madduck@madduck.net>
Place classes mapped onto nodes into __reclass__ key

Signed-off-by: martin f. krafft <madduck@madduck.net>
Enable interpolation/backreferences in regexp-class-mappings

Signed-off-by: martin f. krafft <madduck@madduck.net>
Add note about escaping slashes in regexp

Signed-off-by: martin f. krafft <madduck@madduck.net>
Document class mappings in changelog

Signed-off-by: martin f. krafft <madduck@madduck.net>
Add rebase warning to branch README

Signed-off-by: martin f. krafft <madduck@madduck.net>