Support / add X-Cache and X-Cache-Lookup headers

[blueyed]

Thanks for this nice tool!

After replacing my local Squid instance with polipo, I am missing the "X-Cache" and "X-Cache-Lookup" headers that Squid provides.

I find these useful in general when looking at response headers, and when debugging cache issues in particular.

See also http://blog.lyte.id.au/2014/08/28/x-cache-and-x-cache-lookupheaders/