pypyr.steps.getenv

• add new step pypyr.steps.getenv Whereas pypyr.steps.env raises an error if you are getting an environment variable that doesn't exist, the new envget allows you to specify a default value to use instead. ref #111
• some documentation updates to demonstrate how to use py strings for ternary assignments
• built-in pypes use comment rather than description where output is not meant for operator consumption ref #109
• alias the --loglevel switch with --log, so you can now do pypyr mypipe --log 10 think of the savings just on keyboard wear and tear!