Support more/arbitrary target handlers

[DanCardin]

The idea being that one could define their target handler using some predefined calling convention, and sauce -t foo or whatever to execute a specific subcategory of items; where the executor is defined externally to sauce itself.

The primary motivation being for integration with any 3rd party utility that I cannot reasonably assume all users would use. For example: https://github.com/DanCardin/sauce/issues/30

[DanCardin]

Added file target in v0.8.0, although this is not generic as given above.