Allow for rewriting responses before writing tape

[rchild-okta]

Yakbak is great - it's really easy to setup, and just works. It'd be nice if there was a custom hook to modify the response before writing to the tape - specifically in the case where the response returns references to the proxied server.

For example:

1. Using yakbak server to proxy through to some other server
2. Request made, html returned with references to js/css/img assets that point to the original server url
3. Ideally, I can write a function to replace these references with the yakbak proxy so that they are then captured through yakbak as well.

I submitted a PR with an attempt at this here: #27