Add support for dividend yield about black_scholes_rust

[danielhstahl]

How realistic is having a continuous dividend yield? If it is not realistic, there is a lot of added complexity for no gain.

If it is required feel free to PR.

[rob-keyrock]

Hey, so in practicality dividends will of course be paid out in discrete increments throughout the year. However, this can easily be converted into an annualized continuous yield. Me and extremeandy both have a use case for this so if it is possible it would be fantastic to have the feature added.

[danielhstahl]

Conceptually I'm not sure its "easily converted" but I could be wrong :). Like I said I'm fine with y'all PRing a breaking change involving dividends. It may be worthwhile making the dividend argument an Option type which will make the code slightly more efficient if dividends aren't required.