feature: Collector package

[SuperQ]

Add a collector package that makes it easy to build collector "plugins". This allows users to create exporters by assembling packages from other exporters.

For example, the node_exporter has this feautre:

func registerCollector(collector string, isDefaultEnabled bool, factory func(logger log.Logger) (Collector, error)) 

Some collectors will need custom configuration passed in, or connection information for downstream services (MySQL, PostgreSQL, BIND, etc).