Add `with_route` methods to `IntoPicker` and `PickerPattern` for
setting the route type explicitly when needed, instead of carrying
it as a generic parameter on the trait itself.

Update doc examples to pass `String` instead of `&str` to `convert`

Add `Pickable` implementation for `bool` and the supporting `Matcher`
trait
with utility functions for argument matching.  Expose `pickable_needed`
and
`matcher_needed` modules for downstream trait implementors.

Introduce `or_route` and a `PickerResult` type that carries an optional
route value.  When a flag is absent, the closure given to `or_route` is
evaluated and stored in `error_route`, allowing the caller to exit early
without a default value.

`PickerResult` struct