Add onRune(r string) callback for plugins

Define this function in a plugin (which takes one argument) to be
notified whenever a character is typed, and the character will be passed
as the argument.

cmd/micro/view.go

@@ -298,6 +298,13 @@ func (v *View) HandleEvent(event tcell.Event) {
 			}
 			v.Buf.Insert(v.Cursor.Loc, string(e.Rune()))
 			v.Cursor.Right()
+
+			for _, pl := range loadedPlugins {
+				err := Call(pl+".onRune", []string{string(e.Rune())})
+				if err != nil && !strings.HasPrefix(err.Error(), "function does not exist") {
+					TermMessage(err)
+				}
+			}
 		} else {
 			for key, actions := range bindings {
 				if e.Key() == key.keyCode {