Add `keep_trailing_newline` to configure final endline stripping

This option defaults to False for compatibility with the existing
behaviour.  I've added the option because I expect I won't remember to
keep an extra trailing newline in my templates, and some non-HTML
templates *need* that last newline.

See also:
https://groups.google.com/d/msg/pocoo-libs/6DylMqq1voI/GXTbZJ1Tr-sJ