Add support for proxy protocol in TCP services

[aledbf]

fixes #659

Example:

apiVersion: v1
data:
  "9000": default/http-svc:80:PROXY
kind: ConfigMap
metadata:
  name: ingress-controller-tcp
  namespace: kube-system

     # TCP services
+    upstream tcp-default-http-svc-80 {
+        server                  172.17.0.3:8080;
+        server                  172.17.0.4:8080;
+    }
+
+    server {
+        listen                  9000 proxy_protocol;
+        listen                  [::]:9000 proxy_protocol;
+        proxy_pass              tcp-default-http-svc-80;
+    }

[k8s-reviewable]

This change is 

[aledbf]

ping @donaldguy
Test image: quay.io/aledbf/nginx-ingress-controller:0.158

[coveralls]

Coverage decreased (-0.04%) to 44.299% when pulling 6a4679b on aledbf:proxy-tcp into 24d78ca on kubernetes:master.