Handle configuration merges

[beardedeagle]

With the advent of elixir 1.9 releases, it would be good if sobelow could handle configuration merges:

-----------------------------------------------

Config.HSTS: HSTS Not Enabled - Medium Confidence


HSTS configuration details could not be found in `releases.exs`.

In this case, the following is in prod.exs

force_ssl: [hsts: true],

But it is not present in releases.exs. The reason config merging would be desirable is because if one is using sobelow, it would allow for deduplication of configuration options which currently sobelow forces onto users. Additionally it makes sense for something like this to be an explicit compile time concern and not something someone could come in and edit at runtime via the releases.exs file present in releases.