Add .osm extension for XML

The OpenStreetMap project (openstreetmap.org) uses the .osm extension for the OSM XML format (http://wiki.openstreetmap.org/wiki/OSM_XML).
Merge pull request #1063 from lucaswerkmeister/patch-1

Add .osm extension for XML
Add a multitude of Forth file extensions.
Yes, these are all in use.