Add CATEGORY_OVERRIDES config param for recategorizing transactions

[kevinschaich]

This PR fixes #4 – it adds a list of overrides to handle transactions that are routinely miscategorized by Plaid's servers. Overrides take the following format:

• pattern: JavaScript Regular Expression to test transaction names against
• flags: JavaScript Regular Expression flags (i.e. i for case insensitive)
• category.0: Override for first (top-level) category
• category.1: Override for second (level-2) category

For example, if you want anything matching autopay or e-payment to get categorized as Credit Card Payment, you could add the following line to your .env file:

CATEGORY_OVERRIDES=[{ "pattern": ".*(autopay|e.payment).*", "flags": "i", "category.0": "Transfer", "category.1": "Credit Card Payments" }]