sh: allow passing references through STDIN of a shell script
* lisp/ob-sh.el (ob-ref): Uses ob-ref to resolve the value of the
:stdin header argument.
(org-babel-execute:sh): Use the :stdin header argument.
(org-babel-sh-var-to-sh): Split the bulk of this function off into a
new sub-function.
(org-babel-sh-var-to-string): New function for converting elisp
values to strings that make sense for parsing with sh.
(org-babel-sh-evaluate): Adding "stdin" option to session and
external evaluation options.