Add JSONIFY_END_WITH_NEWLINE config variable

When `JSONIFY_END_WITH_NEWLINE` is set to `True`, `jsonify` responses will be terminated with a newline character.

Defaults to `False` for backward compatibility.

This came up in the context of https://github.com/kennethreitz/httpbin/issues/168
Hmm, on second thought, couldn't we make this more efficient? String concatenation is fast, but it's still creating a new string. Passing an iterable to the response should be faster I think.