Idea for discussion: forkBy

I often find myself in the situation where I want to do the inverse of merge, i.e. split the values of a stream into two different streams. I might be missing some obvious functionality, but this is how I do it today - is there a sleeker way?

``` javascript
let forkBy = (strm, evaluator) => {
let a = _(), b = _()
_(strm).each(x => evaluator(x) ? a.write(x) : b.write(x))
return [a, b]
}
```

Example usage

``` javascript
let isSpecial = str => !!str.match(/^specialcasething/)
let [ specialThings, otherThings ] = forkBy(connectionStream, isSpecial)
```

Is this a pattern you recognize in your coding?

Start with the proposed forkBy entry point and compare it with Highland's existing stream operators, especially merge-related functionality. Review the discussion to determine whether a two-output operator is wanted and what its public API and behavior should be; done requires an agreed design before implementation.

Written by the indexing model from the issue text.