refactor: make the HTTP producer configurable

[paulswartz]

It's better base to build #82 on top of.

[codecov]

Codecov Report

Merging #124 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #124   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files          58       58           
  Lines        1014     1014           
=======================================
  Hits          985      985           
  Misses         29       29           

Impacted Files	Coverage Δ
lib/concentrate/filter/alert/supervisor.ex	100.00% <ø> (ø)
lib/concentrate/filter/gtfs/supervisor.ex	100.00% <ø> (ø)
lib/concentrate/producer/httpoison.ex	100.00% <ø> (ø)
...ib/concentrate/producer/httpoison/state_machine.ex	99.23% <ø> (ø)
lib/concentrate/supervisor/pipeline.ex	100.00% <ø> (ø)

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[losvedir]

👍