Have boot -h differentiate custom task

Hello! 😁

I have introduced boot to a few colleagues now and I feel there should be a clear delimitation in the `boot -h` between tasks that are provided by boot itself and the one provided by the local `build.boot`.
This is very good for discovery of the functionality of the package.

Say I have `repl` and `dev`, I want my developers to use `dev` not `repl` for developing. It is true I can write a description "use this not repl", but nobody unfortunately would read this.

I can go as far as proposing a `:hidden` metadata of some sort that would allow the library maintainer to keep only the tasks that are necessary for usage. No other tasks to pollute the space. Of course they can still be used by savvy devs, but they don't appear in `boot -h`.

Ideas? Comments? 👍 ? 👎 ?

Thanks!