[Feature] Support rx for piping

[eterna2]

Piping can be achieved with rx.

Sample code:

import json

from iotoolz.rx import RxStream
from rx import operators as ops

source = RxStream("http://foo.bar/data.ndjson")

composed = source.pipe(
    ops.map(lambda line: json.loads(line)),
    ops.filter(lambda obj: "key" in obj)
)
composed.subscribe(lambda value: print("Received {0}".format(value)))