For #304: Add $DOCKER_ARGS and docker-args pluginhook to dokku deploy and command run.

Solves https://github.com/progrium/dokku/issues/304 .

Without much code I think it opens up some more advanced options, however the default stays heroku-ish in keeping with the theme of dokku.

Should probably have some documentation somewhere with a pluginhook example so people know they need to read from stdin, append or do whatever they want (with access to $APP as $1), and then echo to stdout so all hooks can manipulate the string in a chain.

I noticed the new docs section so I will add some to there once this is finalised/if it's merged.