Parse JSON login forms for F5

Lots of F5 VPNs don't send normal HTML authentication forms at all, but instead generate forms on-the-fly using JavaScript.

Until now, we've "faked it" by just assuming that the initial login form contains plain username/password fields, but in #512 we have a case where there's a subsequent "challenge" form which also lacks an HTML rendering.

We don't want to actually execute JavaScript in OpenConnect, but as far as we can tell, these JavaScript forms always involve a <script> tag containing a call to a function appLoader.configure(), with a JSON object literal as its sole argument, and that object contains the details of the form fields. Like so:

    <script type="text/javascript">appLoader.configure({"logon": {
        "form": {
            "id": "auth_form",
            "title": "Please log into our F5 VPN",
            "fields": [
                {"type": "text", "name": "username", "caption": "Username", "value": "", "disabled": false},
                {"type": "password", "name": "password", "caption": "Password", "value": "", "disabled": false}
            ]
        }
    });
    </script>

Since we have json-parser as a dependency, let's parse that awful mess, and build our authentication form from it. Yay, JSON.