Domains can be defined as observables

[nhunzaker]

This commit allows a domain to be a simple observable. Observable domains do not have a ledger and can not respond to actions. Their primary use case is to synchronize state with some other source. Like:

1. Window history
2. Websocket state
3. Geolocation

For these cases, you mostly just want to push an action to synchronize state, but that's a ton of boilerplate. Instead, you could do:

import { history } from 'push-state-lib'

repo.addDomain('location', new Observable(observer => {
  observer.next(history.getLocation())

  let listener = history.listen(location => observer.next(location)

  // The return value of an observable's callback is a clean-up function
  // sort of like Domain::teardown
  return () => listener.remove()
})

repo.domains.location.subscribe(location => {
  console.log("The next location is", location)
})

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@945e928). Click here to learn what that means. The diff coverage is 88.88%.

@@           Coverage Diff           @@
##             master   #487   +/-   ##
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  Coverage          ?    98%           
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  Files             ?     29           
  Lines             ?    701           
  Branches          ?    136           
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  Hits              ?    687           
  Misses            ?     12           
  Partials          ?      2

Impacted Files	Coverage Δ
packages/microcosm/src/ledger.js	88.88% <0%> (ø)
packages/microcosm/src/microcosm.js	100% <100%> (ø)
packages/microcosm/src/domain.js	97.91% <100%> (ø)
packages/microcosm/src/subject.js	100% <100%> (ø)
packages/microcosm/src/observable.js	97.72% <75%> (ø)

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