Add support for ignoring false positives in missing foreign key constraint

[chaadow]

Fixes #58

relies on pawurb/ruby-pg-extras#34

the missing_fk_constraints method can now accept an ignore_list parameter, allowing users to skip known false positives.

This is set in the configure block of the gem

RailsPgExtras.configure do |config|
  # Accepts an Array or a comma-separated String of entries like:
  # - "posts.category_id" (ignore a specific table+column)
  # - "category_id"       (ignore this column name for all tables)
  # - "posts.*"           (ignore all columns on a table)
  # - "*"                 (ignore everything)
  config.missing_fk_constraints_ignore_list = ["posts.category_id", "category_id"]

  # Or as a comma-separated string:
  # config.missing_fk_constraints_ignore_list = "posts.category_id, category_id"
end

[pawurb]

@chaadow I could not update this PR to rebase so I'll merge #63