namedlist provides 2 factory functions, namedlist.namedlist and
namedlist.namedtuple. namedlist.namedtuple is similar to
collections.namedtuple, with the following differences:

 * namedlist.namedtuple supports per-field default values.
 * namedlist.namedtuple supports an optional default value, to be used by all
   fields that do not have an explicit default value.

namedlist.namedlist is similar, with this additional difference:

 * namedlist.namedlist instances are mutable.
