Synchronous event handling

[andrewvy]

Building on top of #6.

We could also provide a blocking call, more akin to GenServer.call with timeout functionality if an event hasn't fired after X seconds, for synchronous use-cases. (This is what we do for regular RPC calls anyways.)

{:ok, response} = ChromeRemoteInterface.Page.await(page_pid, "Page.frameStoppedLoading", timeout // 5000)