Add --template option to export command

Used to specify an alternate location to look for exporter's template
files.

Given local files:
   Procfile
   .env
   my-upstart/foreman.conf
   my-upstart/foreman-APP.conf
   my-upstart/foreman-APP-N.conf

One can specify:
   $ nf export --template my-upstart --type upstart

And the templates in my-upstart will be used to generate the upstart
job files.

Closes #23