* lisp/ob-tangle.el (org-babel-tangle): applying optional target-file
argument through integration into the default header arguments
(allowing overwriting through standard header-argument merging), and
allowing a default setting of (:tangle . "no") to be overwritten
when a target-file is specified.
* lisp/ob-tangle.el (org-babel-post-tangle-hook): adding hook which
can be used to call functions from inside of code files tangled by
org-babel-tangle
(org-babel-tangle): added call to org-babel-post-tangle-hook
* lisp/ob-tangle.el (org-babel-spec-to-string): removed
`org-babel-tangle-w-comments', now just use the :comments header arg
to control the insertion of comments on tangling
