Add a "plugin" mechanism to Bond

[gnecula]

We could make Bond more generally useful if it supports a plugin mechanism.

One possible design would be to allow the specification of a plugin object in settings. The plugin can override functions that allow somebody to change how Bond does things.

One use case that came to mind was to add a "before_reconcile" entry point that can pre-process the observation file before comparing it with the reference. I am sure that we will find other use cases (e.g., before_test, after_test, ...)