Allow advanced users to enable request identification via header.

Add support for sending an HTTP header to identify requests.  This provides an
alternate system for negotiation between SPF and default browser requests, but
it adds a restriction on developers: a `Vary` header MUST be returned by the
server to avoid caching problems.  Guard this behavior behind an "advanced"
config flag and provide no default to prevent accidental usage.

A normal SPF request looks like
```
GET /path?spf=navigate
```

By setting `url-identifier` to `null` and `advanced-header-identifier` to
`__type__`, a SPF request would then look like
```
GET /path
Accept: application/json
X-SPF-Request: navigate
```

And the server must then return
```
Vary: Accept
```

Closes #31.