Support for gzip-compressed files in pcap source

[fako1024]

It seems to be quite common to have gzip-compressed pcap files and tools like wireshark support opening them out of the box. Similarly it would be helpful to provide the same feature for the pcap source of slimcap, as it not only extends functionality but also allows for providing compressed test files more easily (e.g. for mock tests in els0r/goProbe#88).