Add a basic CLI

[devongovett]

There have been some requests for a simple CLI for use cases where you aren't using another build tool:

• https://twitter.com/chriscoyier/status/1481350761183420421
• https://twitter.com/WickyNilliams/status/1481378796439752707
• https://twitter.com/Benoit_Chantre/status/1481374449819697152

We could build something super simple using Rust. I think just adding a main.rs should be enough to get a binary target.

[devongovett]

See #52 for followup issue to add more features.