slime-to-lisp-filename-function equivalent functionality

See discusson on:

https://groups.google.com/forum/#!topic/cider-emacs/ooFzP12uNxE

I'm not sure if i covered all location where path translation is required. 

Typical Cygwin usage:

```elisp
(defun cider-from-nrepl (resource)
  ;; why do some windows paths start with a slash? (/f:/dir/file.clj)
  (let ((fixed-resource (replace-regexp-in-string "^/" "" resource)))
    (replace-regexp-in-string
     "\n" ""
     (shell-command-to-string (format "cygpath --unix '%s'" fixed-resource)))))

(defun cider-to-nrepl (filename)
  (replace-regexp-in-string
   "\\\\" "/"
   (replace-regexp-in-string
    "\n" ""
    (shell-command-to-string  (format "cygpath.exe --windows '%s'" (expand-file-name filename))))))

(setq cider-from-nrepl-filename-function 'cider-from-nrepl)
(setq cider-to-nrepl-filename-function 'cider-to-nrepl))
```