Adds filter_request_headers to default parameters

[frm]

Setting filter_request_headers on config.exs is currently working and is a nicer alternative to explicitly setting ExVCR.Config.filter_request_headers("X-My-Secret-Token") in the test files.

This PR adds the option to the default config.exs and to the README file.

[coveralls]

Coverage increased (+0.2%) to 92.936% when pulling ee8c4e6 on fribmendes:master into 5c07f3b on parroty:master.

[parroty]

Thanks!