Minimal hot Observable implementation

Minimal implementation of Observable, like `zen-observable`, but always creating hot observables, like `xstream` and shipping TypeScript definitions out of the box.

Benefit:
- Hot observables are more intuitive
- Making hot observables cold is easy: Just create them in a factory; Making cold observables hot is more difficult
- Way smaller than xstream (bring your own utility functions)

Take care:
- Only run unsubscription code once *all* subscribers unsubscribed
- Might wanna add a `tap` method for handling side effects, even though it can be implemented using `map` in 3 LOC

To Do:
- Check whether callbags are considered a better approach
- Check if xstream does the job already, since it appears to be even smaller than it states in its readme