Combine diffsource, source and confsource; default to '.'

Update rose-stem as discussed with @steoxley and @matthewrmshin; this removes the --diffsource and --confsource options, consolidating them into a single --source argument. This also adds a default setting of --source=. if no source argument is specified It also updates the documentation, fixes a minor bug in cumf (although this should be separated out from rose proper anyway in the future), and renames the Jinja2 variables to start with SOURCE_ rather than URL_. 

The backend (interface between rose-stem, cylc's suite.rc file and fcm-make) needs further thought and work but this update should finalise the user interface for the command line so users have something stable to start using.