Add support for custom proxy headers using a ConfigMap

[aledbf]

fixes #180

[k8s-reviewable]

This change is 

[coveralls]

Coverage decreased (-0.1%) to 44.223% when pulling cbccff8 on aledbf:set-headers into 4eb527d on kubernetes:master.

[coveralls]

Coverage decreased (-0.2%) to 44.183% when pulling cbccff8 on aledbf:set-headers into 4eb527d on kubernetes:master.

[coveralls]

Coverage decreased (-0.2%) to 44.223% when pulling 5cc5669 on aledbf:set-headers into 2045786 on kubernetes:master.

[bigkraig]

This is a huge help for what I am working on, thanks!!

[bigkraig]

@aledbf, any chance we can get the StoreToNodeLister added? I am not sure how else to get the node IPs for my NodePort targets.

I sent you a PR that includes the Node lister aledbf#1

[coveralls]

Coverage increased (+1.4%) to 45.818% when pulling f7a27eb on aledbf:set-headers into 2045786 on kubernetes:master.