Add support for extra payments

[austinmcconnell]

Currently, there is no way to specify extra payments.

Add an extra keyword to the initialization function. This should accept

• Single value (consistent extra amount to be applied to every payment)
• List of tuples containing two values (payment date, and payment amount). This allows for specifying any number of unequal, inconsistent payments.

The second requirement will require rewriting _amortize a bit to check for any extra payments between last payment date and next payment date.

Write test cases for above first. Then implement.