yanglint FEATURE extdata cmd for interactive mode

Implementation of --ext-data parameter for interactive yanglint mode.
diff --git a/tools/lint/cmd.h b/tools/lint/cmd.h
index 071348f..5898cfe 100644
--- a/tools/lint/cmd.h
+++ b/tools/lint/cmd.h
@@ -26,6 +26,7 @@
 
     void (*func)(struct ly_ctx **ctx, const char *); /* Function to call to do the command. */
     void (*help_func)(void);                         /* Display command help. */
+    void (*free_func)(void);                         /* Freeing global variables allocated by the command. */
     char *helpstring;                                /* Documentation for this function. */
     char *optstring;                                 /* Option characters used in function getopt_long. */
 } COMMAND;
@@ -47,6 +48,7 @@
     CMD_LIST,
     CMD_FEATURE,
     CMD_SEARCHPATH,
+    CMD_EXTDATA,
     CMD_CLEAR,
     CMD_VERB,
 #ifndef NDEBUG
@@ -54,6 +56,11 @@
 #endif
 };
 
+/**
+ * @brief For each cmd, call the COMMAND.free_func in the variable 'commands'.
+ */
+void cmd_free(void);
+
 /* cmd_add.c */
 void cmd_add(struct ly_ctx **ctx, const char *cmdline);
 void cmd_add_help(void);
@@ -86,4 +93,9 @@
 void cmd_searchpath(struct ly_ctx **ctx, const char *cmdline);
 void cmd_searchpath_help(void);
 
+/* cmd_extdata.c */
+void cmd_extdata(struct ly_ctx **ctx, const char *cmdline);
+void cmd_extdata_help(void);
+void cmd_extdata_free(void);
+
 #endif /* COMMANDS_H_ */