add purge option to clear database of missing entries

docs/autojump.1

@@ -35,6 +35,8 @@ wrapper function.
 \f[C]
 -a,\ --add\ DIR\ \ \ \ \ \ \ manually\ add\ path\ to\ database
 
+--purge\ \ \ \ \ \ \ \ \ \ \ \ \ deletes\ all\ database\ entries\ that\ no\ longer\ exist\ on\ system
+
 --stat\ \ \ \ \ \ \ \ \ \ \ \ \ \ show\ database\ entries\ and\ their\ key\ weights
 
 --version\ \ \ \ \ \ \ \ \ \ \ show\ version\ information\ and\ exit

docs/body.md

@@ -4,6 +4,8 @@ Options must be passed to 'autojump' and not the 'j' wrapper function.
 
     -a, --add DIR       manually add path to database
 
+    --purge             deletes all database entries that no longer exist on system
+
     --stat              show database entries and their key weights
 
     --version           show version information and exit