Add filter 'by_postal'

[eschwelgin]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Tests pass (bundle exec rake)
• CHANGELOG.md updated

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Current behavior

Lacks ability to search by postal code

Issue Number: Closes #26

New behavior

Added ability to filter breweries 'by_postal'

Does this introduce a breaking change

• Yes
• No

Other information