Add configurable default vi mode (#660)

- `vi_default_mode` added
- supports `normal` and `insert`

res/config.ini

@@ -19,6 +19,11 @@ clear_password = false
 # Enable vi keybindings
 vi_mode = false
 
+# Default vi mode
+# normal   -> normal mode
+# insert   -> insert mode
+vi_default_mode = normal
+
 # The `fg` and `bg` color settings take a digit 0-8 corresponding to:
 #define TB_DEFAULT 0x00
 #define TB_BLACK   0x01