Feature: use <iframe> as redirect mechanism

[dbkaplun]

This would also fix #113 and #114 as the address bar does not update when an <iframe>'s src changes.

New UI:

Current master for comparison:

List of options (let me know if you want different copy):

[dbkaplun]

@jimschubert added screenshots above, please review. Changed the option text to warn about potential security issues. Been using it for a month or so. People could really find this option useful!

[jimschubert]

Thanks for the PR, but as I mentioned this isn't something I'm comfortable adding for multiple reasons:

1. It poses a security risk for which I would potentially be liable
2. It removes standard web behavior
3. I'm fairly certain it violates Chrome Extension agreements
4. (the main one) I am the one that has to answer emails when users ignore things like "potentially insecure" or "power users only". Then, there are users that see things like "potentially insecure" and think it applies to the entire extension. I don't really have time for that.

If you'd like to fork the project and re-release it with the idea me mechanism, I could assist.

[jimschubert]

Er, iframe mechanism. Mobile github doesn't allow editing.

[dbkaplun]

You think people will not be swayed by the warning in the option text? Honestly I think most of those concerns are not too scary but I will change the copy if you're not happy with it. I am using the code in this PR in my Chrome so it would be great if it gets merged but it seems like there is some desire for such a feature so it might be worth it to have this added.