Added heroku-mode-app which user can set to specify an app to the heroku
cli.
Added heroku-mode-execute which will take into account whether
heroku-mode-app has been specified before executing a bash command.
This function will be used by all functions (except heroku-mode-push) instead of shell-command.
Modified heroku-mode-execute to preceed every shell command with "heroku "
Added functions generated by heroku_mode_gen.py