New option to only list unset data

With the -u flag only XMP data that has unset or missing data will be
listed

README.md

@@ -27,7 +27,7 @@ reverse_geolocate.py [-h] -i
     [-l LIGHTROOM FOLDER] [-s]
     [-f <overwrite, location, city, state, country, countrycode>]
     [-g GOOGLE API KEY] [-o] [-e EMIL ADDRESS] [-w]
-    [-r] [-a] [-c] [-n] [-v] [--debug] [--test]
+    [-r] [-u] [-a] [-c] [-n] [-v] [--debug] [--test]
 
 ### Arguments
 
@@ -43,6 +43,7 @@ Argument | Argument Value | Description
 -e, --email | email address | For OpenStreetMap with a large number of access
 -g, --google | Google Maps API Key | If available, to avoid the access limitations to the reverse location lookup
 -r, --read-only | | Read only data from the XMP files and print them out. No LR DB connection is done or any map lookups
+-u, --unset-only | | Only list XMP files with missing data
 -a, --no-autoadjust | | Do not try to auto adjust column width to terminal column width
 -c, --compact | | Do extra compact view
 -w, --write-settings | | Write the Google API key or the OpenStreetMap email into the settings file