Implement config option cert to use a client certificate.

This fixes Issue https://github.com/arthepsy/pan-globalprotect-okta/issues/17 raised by @lvml.

(Note that if you need to specify a client certificate for the `vpn_url`, then most probably
you will also need to give the same certificate to the final `openconnect` call with `--certificate` via the `openconnect_args`)

gp-okta.py

@@ -446,6 +446,9 @@ def main():
 	conf = load_conf(sys.argv[1])
 	
 	s = requests.Session()
+        if conf.get('cert'):
+                s.cert = conf.get('cert')
+        
 	s.headers['User-Agent'] = 'PAN GlobalProtect'
 	saml_xml = paloalto_prelogin(conf, s)
 	redirect_url = okta_saml(conf, s, saml_xml)