Add extra operators to Stream

[wclr]

I think it would be useful to have version of xs of ExtraStream type that would have methods like flattenConcurrently, sampleCombine on it defined so they can be used without compose semantics.

It could be imported from import xs, {Stream} from 'xstream/extra'

Or maybe just add those methods on common Stream type, and add them dynamically on Stream prototype when importing xstream/extra/...

[wclr]

@staltz what do you think about it?

[xtianjohns]

I think this duplicates #128 a little bit, but in general this sounds like it would be helpful. There were some open questions about TypeScript help when patching, but rxjs seems like they've done this at least in part. I know they do module augmentation with import 'extra/operator';.

[geovanisouza92]

I was to comment exactly that: https://github.com/ReactiveX/rxjs/blob/master/src/add/operator/buffer.ts

import 'xs/extra/...'; patching prototype seems to be a simple approach from dev experience.

[wclr]

I know about #128, but the problem with installing some operators is just with typing. I'm not aware how Typescript could allow having progressively extended class definitions by importing some modules.

So I propose to create a separate class that extends common xstream with all the extra methods seem to me as the only normal solution. ES6 tree shaking is here to solve this so "important" front-end bundle size question, for those who really care (I don't care much).