Feat: Add support for YouTube videos as a thumbnail type

**What type of feature request is this?**
- [X] Mantle doesn't support a Roblox feature I would like to use
- [ ] Mantle CLI improvement
- [ ] Other

**Describe your problem**
Add support for YouTube videos as a thumbnail type
**Describe the solution you'd like**
Define a youtube link in mantle.yml for the file path