more default order options

[johnnychen94]

Currently, it uses case-insensitive alphabetic order for demos, it might be helpful to allow users to specify a more "meaningful" order preset. For example:

{
    "order": "alphabetic"
}

This doesn't break the current syntax, as the previous order is defined as a list (not a string):

{
  "order": [
    "basics",
    "julia_demos"
    ]
}

---

Here are presets that I guess would be useful:

• "Alphabetic": case-sensitive alphabetic order.
• "xxx-rev": reversibly order by preset xxx.
• "modified_date": order by git commit dates

Suggestions for different order preset are welcomed.