Add an option to remove unused requests from cassette

[danielnsilva]

This PR adds an option to remove previously recorded interactions if they are not used. I believe this feature can address the issue discussed in #208. We don't need to focus on ignoring all requests in the cassette file, but instead, we can write to file only replayed interactions and new ones. If no previous request is used, VCR will save only new interactions.

There is a similar feature in Ruby VCR.

Replaces #677