Separated cli and postcode logic

[antoni-devlin]

Moved functions around so the postcode validating, converting, and formatting logic lives in postcodes.js.

Cli.js now contains only functions required to interact with postcode.js using the command line.

[antoni-devlin]

• The title on this commit is good, but could we expand on the "why" here in a deeper message. You might want to lave yourself even more detail on what benefits that kind of separation brings? If you put this down for a year and had to pick it up later and remind yourself, what could you say in this commit to help you remember why you did this?

@huwd Updated the commit here to be more descriptive by:

• using git rebase -i HEAD~7
• changing pick to reword in the commit list
• changing my commit message
• running git push --force-with-lease origin refactor-12May

Looks like it's worked as expected, so I'll merge this :D