Add custom coercion

[jackfirth]

This PR adds support for a #:coerce keyword that allows a user to supply their own function for converting the environment variable string value into a value of the declared type. When provided, it overrides the default coercion for the declared type if there is one.

(: octal-string->integer (-> String Integer))
(define (octal-string->integer s)
  (cast (string->number s 8) Integer))

(define-environment-variable an-octal-integer : Integer #:coerce octal-string->integer)

Closes #4

[jackfirth]

Closing due to age