Add support for macros

Closes #270

CtrlU to toggle recording and CtrlJ to playback.
You can also rebind using the "ToggleMacro" and "PlayMacro"
actions.

Note that recursive macros are not yet supported.

runtime/help/keybindings.md

@@ -63,6 +63,8 @@ you can rebind them to your liking.
 	"CtrlQ":          "Quit",
 	"CtrlE":          "CommandMode",
 	"CtrlW":          "NextSplit",
+	"CtrlU":          "ToggleMacro",
+	"CtrlJ":          "PlayMacro",
 	
 	// Emacs-style keybindings
 	"Alt-f": "WordRight",
@@ -177,6 +179,8 @@ PreviousTab
 NextTab
 NextSplit
 PreviousSplit
+ToggleMacro
+PlayMacro
 ```
 
 Here is the list of all possible keys you can bind: