Add HTTP output handler

[mfdii]

Adding a simple HTTP handler that allows you to post JSON output directly to a HTTP endpoint instead of the current method of using the program output and curl.

This was implemented in C++ and the outputs.lua calls back into C++ to handle the the http post using libcurl.

This can be enabled by setting the http_output option in falco.yaml to enabled: true.

http_output:
  enabled: true
  url: http://some.url/you/post/to

[mfdii]

@mstemm this is failing on gcc-6, just fyi. I know you're working on it

[mstemm]

Looks good. Since the http request is blocking we should note this in the docs/wiki. We'll fix the whole output channels to be non-blocking in a (soon) later PR.