Add channel extensions - RFC

[oriweingart]

Initial code sample:

const few = require('few');
const channels = require('few/channels');
const request = require('request');
​
few(function* () {
const channel = channels.open();
for (let url of [GITHUB_STATUS, ASANA_STATUS, PAGERDUTY_STATUS]) {
request.get(url, channel.put());
}
// Do other things...
while (yield channel.hasNext()) {
const response = channel.next();
console.log(JSON.parse(response));
}
});