Basic framework for class mappings

Each node's classes are now pre-initialised depending on the
classes_mapping key in the config file, e.g.:

  class_mappings:
    '*': default
    /^local/:
      - local

This will cause all nodes to get the 'default' class before anything
else, and it will cause all nodes whose name start with "local" to get
assigned the 'local' class.

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

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>
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>