Add support for -t/--template option to export

This is a honcho analogue of the corresponding options to `foreman
export` (See http://ddollar.github.io/foreman/#EXPORTING)

This lets you specify a custom Jinja template to be used when exporting
to a supervisord config file.

Fixes: GH-88

See: https://github.com/nickstenning/honcho/issues/88