[Request] make ".parallel" to run N concurrent functions.

First of all, thanks a lot for Nimble! :)

I would like to suggest a feature: right now the "_.parallel" function definition is
"Parallel will run a list of async functions _ALL_ at the same time."

It would be awesome to have a simple mechanism (queue based ?) to specify how many functions to run in parallel instead of ALL.

For example, imagine what it will happen when each function triggers an HTTP request.