add redactions to VCR

[aksonnic]

Context

Typically there are lots of private values in network request/response that should not be saved to storage, and potentially be committed as part of a cassette. This PR allows users to specify redactions to be applied before/after storage. In all other respects the library works as it did before.

What has been done

• added ability to specify redactions to be applied
• added scrubbing/unscrubbing just before a recording is recorded/played back
• tests

How to test

• run unit tests

Notes

• I don't really like the asymmetry between scrub($request, $response) and unscrub($recording). It's like that because each direction instantiates Request or Response objects at different times, and was an attempt to prevent needless array<->object conversions, but maybe it's not worth bothering.
• Scrubber/scrub/unscrub perhaps isn't the best naming

[Justintime50]

Can we get eyes on this? This is an absolute must for security and a blocker before we could use it on our projects.

[TheFehr]

@aksonnic I added your changes to my own fork, as I also needed a fix for a small curl bug. Can you give me an example of how to use it? I see you added tests, but there you create your own Cassettes. Is that the only way? Is this no compatibly with the "default" use of

VCR\VCR::turnOn();
VCR\VCR::insertCassette('some_name.yml');

/** The tested code **/

VCR::eject();
VCR::turnOff();

Thanks a lot