Fix #2856: Add hook for determining whether to override resolver implementation

[malcolmohare]

Proof of concept fix for #2856 that doesn't involve making a breaking change.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 74.837% (-0.01%) from 74.848%
when pulling dcb707b on malcolmohare:mo/resolver-plugin-fix
into e61a720 on 99designs:master.

[StevenACoffman]

Closing in favor of #2886 (although I generally prefer non-breaking)