Document variadic arguments and recursive calls

[aradi]

Improves documentation on variadic arguments and explicitly mentions recursive calls using the Horner-scheme as demonstration.

Closes #8 Closes #21

[ivan-pi]

In the Horner example it might be good to add another pair of brackets in case of negative coefficients:

  #:def horner(x, a, b, *args)
  #:set res = "({} * {} + ({}))".format(a, x, b)
  #:if len(args) > 0
    #:set res = horner(x, res, args[0], *args[1:])
  #:endif
    $:res
  #:enddef

[aradi]

Done (ead4ade), thanks!