Add support for PROXY protocol

[jordansissel]

Forked continuation of #34.

[jordansissel]

@nneul I've restructured some of the tests from #34 and things are passing now for me.

[jordansissel]

(Caveat, this PR will only likely work against Logstash 5.2 or master branches because Logstash 5.1.1 doesn't have elastic/logstash#6309)

[nneul]

---

@nneul did you intend to have this not set proxy_host or proxy_port fields? The TCP input does this :

https://github.com/logstash-plugins/logstash-input-tcp/blob/master/lib/logstash/inputs/tcp.rb#L183-L188

No, that was not an intentional exclusion. Simple enough to add.

[jordansissel]

@nneul ok cool, I will add it.

[jordansissel]

This PR is blocked until Logstash 5.2 is released. Logstash 5.0 and 5.1 both have a bug fixed in 5.2 branch.

[nneul]

@jordansissel Any update on merging this as it looks like 5.2.1 was already released?

[jordansissel]

@nneul This is on my todo list for after I recover from Elasticon and this week's meetings.

[jordansissel]

Woo, CI passing.

[ph]

@wHack LGTM, Can you update the changelog + bumping the version?

One of the build job on travis was failling, It was a bundler caching issue, after clearing the cache everything went green!

[nneul]

Anything else that needs to be done before this can be merged?

[elasticsearch-bot]

Jordan Sissel merged this into the following branches!

Branch	Commits
master	48b08a2, c759ae8, 08b9ab0, bee7a1a, 9cb1782, 7f2c73a, d732320, a6a23f4, 4591966

[jordansissel]

I will publish this as a plugin soon. I have some new work I need to add to this plugin before publishing.