Allow partial configuration by merging local with default configuration

By merging the configuration read from `config/vcr_cable.yml` into the `DEFAULT_CONFIGURATION`, I have enabled "partial" configuration i.e. setting only the values you want to change from their defaults.

I also simplified the test suite by using `test` as the non-vcr_cable environment and `development` as the with-vcr_cable environment, which matches up with the description of the gem and the provided default configuration.

Thanks for making this cool resource. I look forward to your feedback!

**jmm**