Add markup for "on-going" sub-tasks

On an issue description we can use markup to make a list of items with checkboxes. I informally call these "sub-tasks".

These are created by inputing something like this:

- [ ] subtask description

And these are rendered as checkboxes which are eighter ticked or not, like this:

• subtask description

Sometimes I feel like it would be useful to have an intermediate state for these checkboxes, that might be represented by markup such as any of these:

- [*] an asterisk
- [-] a simple dash
- [!] an exclamation point
- [?] or perhaps anything else...!

And these should be rendered in a different way, so that you convey the idea that someone is working on that subtask. Like an "on-going" task.