New filter: NamedTuple

[todofixthis]

Implement a new filter that, given a collections.namedtuple type, attempts to convert the incoming value into that type.

Examples:

Colour = namedtuple('Colour', ('r', 'g', 'b', 'a'))

f.NamedTuple(Colour).apply([96, 96, 192, 1.0])  # Colour(r=96, g=96, b=192, a=1.0)
f.NamedTuple(Colour).apply('1234')  # Colour(r='1', g='2', b='3', a='4')
f.NamedTuple(Colour).apply({'r': 96, 'g': 96, 'b': 192, 'a': 1.0})  # Colour(r=96, g=96, b=192, a=1.0)

f.NamedTuple(Colour).apply([0])  # FilterError (not enough values)
f.NamedTuple(Colour).apply([0]*5)  # FilterError (too many values)
f.NamedTuple(Colour).apply({"c": 192, "m": 36, "y": 192, "k": 68})  # FilterError (wrong keys)
f.NamedTuple(Colour).apply(42)  # FilterError (wrong type)