Put Plugs in runtime mode in :dev, :test to speed up recompilation

Merged Alex Gleason requested to merge plug-runtime-dev into develop

This is another recompilation improvement to speed up recompilation in dev and test.

This is not a magic bullet to the problems in #2651 (closed) (eg it doesn't fix recompilation speed of views), but it will make it a faster to edit plugs.

More info: https://dockyard.com/blog/2018/02/12/what-s-new-in-phoenix-development-february-2018

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