Add --no-warn-https option

to specify hostnames to not emit HTTPS warning for.

```
$ pip install --no-warn-https devpi.mycompany.com -i http://devpi.mycompany.com pyramid
```

This will suppress the following warning that would normally be output in yellow:

```
http://devpi.mycompany.com uses an insecure transport scheme (http).
Consider using https if devpi.mycompany.com has it available
```

I can add tests if the idea for the feature is deemed sound and useful.
Thanks! While looking at it and applying your changes, I think I earned a few pedant points of my own, as I realized that the comment isn't even accurate. We're not skipping the check, as it's already happened. We're just suppressing the warning. It's also a little dubious to mention HTTP or HTTPS because the method name and implementation seems to avoid specific protocol references and prefers general terms like "insecure schemes". I should probably follow suit.