Integrate chained ConfigurableValues for global defaults

*(Using a not-yet-available prerelease version of `Hudl.Config`).*

Fixes #33.

Breaker, metrics, pool, and fallback configuration can have default global values provided via config (instead of the hard-coded literal defaults), which allows greater flexibility to tune the defaults at runtime or in an environment-specific way.

Example (using a circuit breaker config); values are used in this order:

1. `mjolnir.breaker.<breaker-name>.thresholdPercentage`
2. `mjolnir.breaker.default.thresholdPercentage`
3. Literal default from `CommandContext` (`50`).

**Potential breaking change:** Any breakers or pools that had previously been named `default` will and received specific configuration values will cause those values to get used over the defaults in the code.