Add 'genpoi' command

This change will add new command 'genpoi', similar to 'list', that outputs the current dungeon locations in the POI format used by the OverViewer interactive mapping tool, generating a valid config.py that can be used for map generation.  I find this handy to help locate the various structures in a convenient interactive map.
To do this, a new flag is added to listDungeons() to output in OverViewer format, and this is linked to new options in the parser.
OverViewer can be found at http://overviewer.org/
Improvements could include passing a location for the generated POI data, adding a 'quiet' option to suppress progress messages, merging with 'list' as a modifier rather than a separate command