Add option for users to define indicators in their own crate

[jpmckinney]

Instead of explicitly listing all indicators in the add_indicators! macro call, we can use the inventory crate, like at dtolnay/inventory#21 (comment), so that each trait object registers itself.

Putting as nice-to-have as we have no demand for this yet.

If it makes our own code cleaner, we might consider doing it anyway.

Blocked by dtolnay/inventory#51