Lazily flatten middleware

[PlasmaPower]

This is important as it allows for later modification, fixing a breaking change. Resolves #66, resolves #67.

[coveralls]

Coverage remained the same at 100.0% when pulling 05158ca on PlasmaPower:lazy-flatten into 2276496 on koajs:master.

[jonathanong]

why are you editing the middleware?

[PlasmaPower]

I'm not, but we've already broken at least 3 user's programs. I think that can occur if you app.listen before you app.use. At this point we should either version it as a breaking change, revert it, or merge these PRs.

[PlasmaPower]

Closing due to reverting variadic arguments instead. May be reopened in the future.