yanglint FEATURE extdata cmd for interactive mode

Implementation of --ext-data parameter for interactive yanglint mode.
diff --git a/tools/lint/completion.c b/tools/lint/completion.c
index 2eab01c..db2704d 100644
--- a/tools/lint/completion.c
+++ b/tools/lint/completion.c
@@ -413,6 +413,7 @@
         {CMD_PRINT,       "-o",    linenoisePathCompletion, NULL},
         {CMD_PRINT,       NULL,    NULL, get_model_completion},
         {CMD_SEARCHPATH,  NULL,    linenoisePathCompletion, NULL},
+        {CMD_EXTDATA,     NULL,    linenoisePathCompletion, NULL},
         {CMD_DATA,        "-t",    NULL, get_data_type_arg},
         {CMD_DATA,        "-O",    linenoisePathCompletion, NULL},
         {CMD_DATA,        "-R",    linenoisePathCompletion, NULL},