Add support for add_headers

[n1koo]

Add support for add_headers globally. Uses similar pattern as custom proxy headers in #246 .

Could be taken further by adding support for annotations and adding this per location, but global is what our usecase needs right now

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[k8s-reviewable]

This change is 

[n1koo]

CLA signed by having trouble linking it to GH, lets see if it worked now

[coveralls]

Coverage decreased (-0.1%) to 46.994% when pulling 551410a8cce4f8c0a05a83aae5d42afd664dbf83 on n1koo:add_headers_support into b7bf783 on kubernetes:master.

[jonpulsifer]

@aledbf any opinions?
@n1koo can you see if you can get the CLA sorted out? that did it I guess

we're looking forward to this too 👍

[aledbf]

@jonpulsifer this PR needs a rebase

[n1koo]

Sorry about that, totally forgot this PR altogether. Rebased and CLA happy now :)

[coveralls]

Coverage decreased (-0.06%) to 44.323% when pulling 02832de on n1koo:add_headers_support into c1e7c7a on kubernetes:master.

[aledbf]

/lgtm

[aledbf]

@n1koo thanks!