unmap a binding or delete a command when expression is empty

With this commit `map`/`cmd` keywords are also used to remove a
binding/command by leaving the expression part empty (e.g. `map j`
removes keybinding for `j` or `cmd trash` removes `trash` command).

Mentioned in #16.

eval.go

@@ -106,10 +106,18 @@ func (e *SetExpr) eval(app *App, args []string) {
 }
 
 func (e *MapExpr) eval(app *App, args []string) {
+	if e.expr == nil {
+		delete(gOpts.keys, e.keys)
+		return
+	}
 	gOpts.keys[e.keys] = e.expr
 }
 
 func (e *CmdExpr) eval(app *App, args []string) {
+	if e.expr == nil {
+		delete(gOpts.cmds, e.name)
+		return
+	}
 	gOpts.cmds[e.name] = e.expr
 }