Allow filepath autocompletion in plugins

closes #858

apidocs/c/profapi.h

@@ -93,6 +93,13 @@ Remove all values from autocompletion for a command, or command argument.
 */
 void prof_completer_clear(const char *key);
 
+/**
+Add filepath autocompletion for a command, or command argument.
+
+@param prefix the prefix from which filepath autocompletion will be triggered
+*/
+void prof_filepath_completer_add(const char *prefix);
+
 /**
 Send a desktop notification.
 @param message the message to display in the notification