Make it possible to return PSGI-style streaming response

This is an proper version of my earlier pull-request with documentation and tests.

Example:
```perl
get '/streaming' => sub {
    # use my app infrastructure to make database lookups, session checks etc.
    # then return a streaming app
    return sub {
        my $respond = shift;
        my $writer = $respond->( ... );
    }
};
```