ob-tangle: :comments header argument can now tangle surrounding text

ob-tangle: :comments header argument can now tangle surrounding text

This commit introduces a new set of :comments header arguments
- no :: retains its behavior of not tangling any comments
- yes :: retains its behavior of wrapping the code in links back to
         the original org-mode file
- link :: is synonymous with "yes"
- org :: does not wrap the code in links back to the original org
         file, but does include preceding text from the org-mode
         file as a comment before the code block
- both :: turns on both the "link" and "org" options

* lisp/ob-tangle.el (org-babel-tangle-pad-newline): can be used to
  control the amount of extra newlines inserted into tangled code
  (org-babel-tangle-collect-blocks): now conditionally collects
  information to be used for "org" style comments
  (org-babel-spec-to-string): now inserts "org" style comments, and
  obeys the newline configuration variable when inserting whitespace
* doc/org.texi (comments): documenting the new :comments header
  arguments