Expand number of valid list-characters (refs #789).
Expand number of valid list-characters (refs #789).

With:
- `- [ ] `
- `- [x] `
- `* [ ] `
- `* [x] `
- `+ [ ] `
- `+ [x] `

pbek/qmarkdowntextedit,pbek/qmarkdowntextedit,pbek/qmarkdowntextedit