Add authentication support

[Manfred]

Most applications use some form of authentication so it's probably useful to add some sort of mechanism to PerfCheck to select a role and/or user for authentication.

I think we want to pass the role as a string so the target application can resolve it in a useful way. In the context of this issue I don't care how the target application does this.

perf_check --user-role admin /manage/users

For selecting a specific user we also want to accept a string.

perf_check --user [email protected] /dashboard
perf_check --user 5543453 /dashboard

We can also limit the API surface by forcing users to encode their user scheme into some sort of serialized form.

perf_check --user company:12,user:5 /dashboard
perf_check --user role=admin /admin/board

Internally we can call this options.user or option.authenticated_user for extreme clarity. Eventually it should be replaced by options.cookie or options.headers.