Strong debugging integration

[ccmcintyre-kingsds]

In service of more effective problem diagnosis and resolution, for both Bifrost developers and third-party users:

• Block processes on debugger, for both Bifrost internals and for user scripts being run.
• Provide port for process introspection in the presence of a --debug flag argument.

Relates to #94 (Strong error handling integration).
Relates to #95 (Strong testbed integration).

[ccmcintyre-kingsds]

Thank you @wesgarland for suggesting and emphasizing these debugging approaches; implementing these will be prioritized in the current feature milestone.