First of a few refactors to allow multiple clients / frontends for
Delve. Current implementation now uses a cli client, but conceivably we
could have an http or socket based client as well.

Areas that need improving:

* Code cleanup
* Promote breakpoints back out of thread context
* Fix potential bug in "Next" implementation, when thread contexts
  switch

Implement feature that enables the debugger to be able to compile, run
and attach to a program all in one simple step.

Benefits of this CL:

* Commands no longer rely on closured proc struct
* No two-step process for basic command initializaion