feat: add Vim-style I and A and move redo to U

src/main.rs

@@ -6,8 +6,9 @@ const HELP: &str = "vamp — modal editor with isolated Ruby plugins
 
 Usage: vamp [--config PATH | --no-config] [FILE]
 
-Normal: i/a insert, o/O new line, h/j/k/l move, 0/$ line ends,
-        x delete, u undo, Ctrl-r redo, : command
+Normal: i/a insert, I/A insert at first nonblank/end, o/O new line
+        h/j/k/l move, 0/$ line ends, x delete
+        u undo, U redo, : command
 Insert: Esc returns to normal
 Commands: :w [PATH], :q, :q!, :wq [PATH], :plugins, :PLUGIN [ARGS]