First cut of refactor

[midwire]

• I wrote specs to cover new or modified code in this PR
• I ran rake spec locally and ALL specs pass

Related Issue: #5
Description: First cut of refactor. I have not written specs yet but I tested it locally with allCountries and US. It is much faster, thanks to @firien’s PR #1 with the idea to use in-memory SQLite databases.

Here’s an overview of what I did:

• Made it a gem with a command line executable bin/free_zipcode_data
• Support user switches for various options including custom table names
• Use Kiba for ETL
• Use in-memory SQLite database to create the tables, then save it as a file on disk
• Separate concerns for each table and in general

Interested in comments or ideas for improving it.

[cmckni3]

LGTM