Add a warning when route.start is called multiple times

[chasenlehara]

done-autorender automatically calls route.start() for you when it sets up your app’s view model. If you follow the can-route docs and call start yourself, your app won’t work because done-autorender will create a new route.data and call route.start() itself.

I think there are a couple possible warnings we could show that would help users understand this issue:

1. Warn if route.start() gets called a second time without first having been torn down.
2. Have done-autorender check whether start has already been called, and provide an error/warning if it has.

[chasenlehara]

I’m closing this in favor of #153