Introduce spy or stub options to Hoverfly

[mogronalol]

This comes from this issue in Hoverfly-Java.

With Mockito in Java, you have the ability to Spy on real objects. This means instead of having a pure mock, you have the real object. In this case, individual methods are overridden to become stubs, leaving the the non-stubbed methods to work as normal.

This seems like something Hoverfly could do, but for http. So something like:

hoverctl simulate --stub // The same as how it currently behaves
hoverctl simulate // The same as above because stub is default
hoverctl simulate --spy // When there isn't a match, call the real service

This is a valid use case, in some situations. The one given was running the external dependency locally or as part of your build, and only wanting to change the behaviour of specific endpoints.

[tommysitu]

Spy mode has been introduced in this PR: #680

[tommysitu]

Thank to @3sunnysideup, Hoverfly v0.15.0 has a new spy mode.