Use allocate instead of new in rack app

Roda by default does not define initialize for the instance scope,
so calling new is the same as calling allocate, just slower.

It may be faster to use new(env).call(&block) if #call doesn't
call #_route, but that would change the API, and in my testing it
isn't conclusively faster in all cases.
Add some notes about flushing in subtemplates to the chunked plugin
Add some specs to get back to 100% coverage

Only significant code change is that the Roda.new in the middleware
plugin now requires an argument.  Now that Roda uses allocate
instead of new for creating new instances, new should now only be
called in a middleware setting.
Add note about proxy http versions to chunking plugin doc