fix: fire compound interest w/ start balance

[ginatrapani]

Add a custom function that calculates how many months it will take to reach a goal balance given compounding interest with a start balance, monthly investments and an annual rate of return.

Call it in the fire calculator to calculate time till fire.

-- I needed a different compounding interest formula than the expressions we have in current calculators, and I wanted it to be easier to read/reason about, as I'm just a simple cavewoman. There's probably a better place to actually put custom parser functions, other than in smarter-text.js. Happy to move it elsewhere!