How to start/end reactor? #109

Hi, thank you for providing very awesome async library for Ruby.
I have an issue on using Async in my browser-automation library.

I understand this async library requires root reactor session with Async { ... } or Async::Reactor.run{ ... }.
Sometimes it is hard to define an async session using block, for example:

browser.on(:start) do
  # start async reactor session here
end

browser.on(:exit) do
  # end reactor session here
end

Can we define Async reactor session without block?