Create output plugin architecture

All outputs are now plugins.  This moves the old screen and email
outputs to use the new plugin architecture.  Plugins need to only
minimally implement what is in output.py.  It should define the
attribute name and implement the function do_report(list).
updates to constants.py

Take in get_config_value() from utils.py.  Update handling of SMTP
constants and add DEFAULT_OUTPUT_PLUGIN_PATH.
Updates to utils.py

More helper methods in formatting task/playbook for display
format_playbook_report() only works on one playbook at a time now.
Update ansible-report for output plugins

Support use of output plugins
Add option -e, --extra-args to pass arguments to plugins
update comments for output plugins on usage
update handling of data files
Update README.md
update README.md for sqlalchemy info