adding a new global tangle-mode header argument
* doc/org.texi (Top): Documentation for new tangle-mode header argument.
(Specific header arguments): Documentation for new tangle-mode header
argument.
(rownames): Documentation for new tangle-mode header argument.
(tangle-mode): Documentation for new tangle-mode header argument.
* lisp/ob-core.el (org-babel-common-header-args-w-values): Adding the
new :tangle-mode header argument.
(org-babel-read): Read values starting with a "#" character as emacs
lisp.
* lisp/ob-tangle.el (org-babel-tangle): Use the new :tangle-mode header
argument.
* lisp/org-pcomplete.el (pcomplete/org-mode/block-option/src): Use the
new :tangle-mode header argument.