Add support for if/unless validator options

[knovoselic]

This PR adds support for testing validators which are triggered only under a certain condition. The change supports if/unless options with symbol or lambda parameters as the conditions.

[dblock]

Definitely need to fix CI first.

[dblock]

@knovoselic interested in helping out with maintaining this library? If @rodrigopinto doesn't object I can add you here and rubygems.

[dblock]

Maybe you can finish #216 too? :)

[knovoselic]

@dblock thanks! I'm not sure if I'll have enough time to help with maintenance of the library. Let's see if I can find some time next week to take care of #216.

[johnnyshields]

FYI, this PR broke semver of this gem as it was released as 4.2.0 (tiny version bump)--it caused many of my existing test cases to fail.

I have merged a PR here: #249 which now gates it behind a .check_conditions method.

I am lukewarm on the idea of making this "check_conditions" logic to be the default behavior. If we were to do it, it would need to (a) be done in a major release, i.e. 5.0.0, and (b) we would have to handle it very carefully. Read this comment for details:
#249 (comment)

[dblock]

Agreed @johnnyshields I think a 5.0 with default behavior makes sense.