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.