Add a scan operator

[dariomannu]

Describe the solution

Sometimes overlooked, including in the current WICG/Observable proposal, the scan operator (a progressive reduce that emits over time, not just on completion) is fundamental for implementing even basic functionality such as a click-counter button, which is like the 'Hello world' of reactive programming. Please consider adding it.

[hntrl]

will take some time for me to get to this, so would happily accept a PR if you have time!

[dariomannu]

Have you considered joining the Hacktoberfest? It's starting right now; it could be a great opportunity to get some extra help and visibility for the project...