Add sendMessage as a parameter of the onOpen handler

[bchilcott]

It looks like the sendMessage paramter was added to onOpen as part of #32 but has since been removed. Could we put this back, or is there a better way to send a message in the onOpen handler?

[michaelboyles]

Is your use-case there same the one in that issue? You are memoizing the options? It's not necessary. I posted a comment over there.