Add ssl verify flag

[slash-zero]

Adds new command line arguments --no-verify-ssl and --verify-ssl (is default behavior). You can now pass --no-verify-ssl and the jira requests won't fail instantly with self-signed or somewhat invalid certs.

[pawelrychlik]

@slash-zero thanks for contribution. It took me a while because of two reasons:

• the cut between this PR and https://github.com/pawelrychlik/jira-dependency-graph/pull/22 was not so clean. You built the SSL flag on top of the previous branch. So I had to merge both at once.

• I allowed myself to do a small clean-up (unfortunately it ended up being a part of the merge commit https://github.com/pawelrychlik/jira-dependency-graph/commit/53e8190c386d62da19a3c64602ed3ef2503d91ba), mostly related to the command-line parameter processing of SSL flag. In my view, the way argparse works with boolean parameters leaves a lot to be desired, but I'd keep the parameter names aligned (i.e. a single param --xyz to switch a feature on/off), rather than introduce a whole new way of defining parameters (--xyz and --no-xyz).

I might consider refactoring the parameter parsing in a way you suggested, but I'd rather migrate all commands at once, than go one by one.