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.
I think I'm going to change it to "hostnames for which we won't warn".