Support for let expressions

[Atreyagaurav]

Need to make the format-args-let function that can format the let expression like the following.

(defun runoff-depth (cfs)
(let ((area-miles 0.2)
      (time-secs (* 1 60 60)))
  (/ (* 12 cfs time-secs) area-miles (expt 5280 2))))

(runoff-depth 60)

Into something like:

Given (area-miles=0.2;time-secs=3600):
runoff-depth(cfs):\frac{12 \times cfs \times time-secs }{area-miles \times 5280^{2}}