Expose the Signal.Address which StartApp uses internally.

[rgrempel]

This PR would address the scenario identified in #29.

Note that client code would not normally need to access the Signal.Address, since it would normally be sufficient for client code to supply a Signal action as one of the Config.inputs when calling start. So, this PR would address an unusual or rare situation.

So, part of the issue is how start-app intends to deal with unusual or rare situations, as discussed in #29.

• One option would be to say: if you have an unusual or rare situation, simply make a local copy of start-app and modify it as needed. In that case, one would expect that this PR might be rejected.
• Another option would be to expose more of the internals of start-app, so that clients can deal with unusual or rare situations without having to modify start-app itself.

I don't have a strong preference as between those two options.

[rgrempel]

I ended up figuring out how to make elm-route-hash work with an unmodified StartApp -- see

rgrempel/elm-route-url@887b033

So, I'll close this PR -- there still may be reasons for exposing the address, but I no longer have an example of one.

[amitaibu]

👍 I love your detailed explanations, it's really valuable - thanks!