Argument: Ignore SSL certificate errors

The example in the README, remotezip -l "http://thematicmapping.org/downloads/TM_WORLD_BORDERS-0.3.zip", currently fails with an SSLCertVerificationError. In general, it would be a good idea to include an option that allows for insecure connections to a server.

We could follow curl's argument design and use -k and --insecure to denote this.