Mortgage/Loan calculator goodie

Inspired by my own experience using mortgage calculators and also based on the duckduckhack idea: https://duckduckhack.uservoice.com/forums/5168-ideas-for-duckduckgo-instant-answer-plugins/suggestions/3304604-loan-calculator 

I think off the bat this satisfies what many people would be interested in calculating and uses a somewhat flexible syntax. I could also add taxes and PMI to the equation. Total amount paid might be interesting to include in the results although I'm not sure if people care about that number or not (besides being something that gives them nightmares).