implement get fps

[Luca00casati]

there is doc to add, i will added tomorrow

[emoon]

Thanks for the PR! Now when you have added this I think it makes sense to be able to just get back the regular time as well instead of just fps. Do you think you can add that while at it?

[Luca00casati]

so something like this?
double GetTime(void); // Get elapsed time in seconds since InitWindow() stolen from raylib

[emoon]

I think perhaps get_delta_time() here would be best as it's the delta that has passed between the updates.