ENH: Improve ExtensionDescription API

Add ability to directly construct an ExtensionDescription from a file
path. Note that unlike read() this takes the path to a description file,
not a directory containing a (single) extension description. This is
intended to be used to read extension descriptions from the index
repository.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22970 3bd1e089-480b-0410-8dfb-8563597acbee