[feature request] support multiple-level menu

[iamqiz]

(Please fill out the issue template with your details)
i use GM_registerMenuCommand to create menu,
but the menus are flat ,they have same levels,
i hope they have different levels, so they can be organized into a tree

Expected Behavior

multiple level

Actual Behavior

Specifications

• Chromium: (i.e. 64.0.3282.140)
• TM: (i.e. 4.5.5648)
• OS: (i.e. MacOS 10.13.4)