Added the ability to have a preferred TW account, very useful when th…

[julie-mindcapers]

Problem addressed

User has multiple TastyWorks Accounts and accounts[0] is a closed account.

Solution

Add TW_PREF environment variable to store the user's preferred account; add fuction get_pref_account to parse the accounts list and return the preferred account if it exists, otherwise it returns accounts[0] just as before.

Checklist

• PR commits have been squashed Unknown, this is me dipping my toe in

• All tests pass Unknown, this is me dipping my toe in

• Code adheres to contributing guidelines I did install and run flake8 on my changes until there were no more warnings.