babel: evaluation of code blocks now requires confirmation

babel: evaluation of code blocks now requires confirmation

* lisp/babel/ob.el (org-confirm-babel-evaluate): variable used to
  control evaluation of code blocks, default value it t, meaning all
  code block evaluation requires confirmation

  (org-babel-confirm-evaluate): function used to request confirmation
  of code block evaluation from the user

  (org-babel-execute-src-block): this function is the single point of
  entry for evaluation of code blocks (whether initiated through lob
  call, through direct code block evaluation, or as part of file
  exportation).  Every time this function is called it will now
  request confirmation from the user.  The newly added
  `org-confirm-babel-evaluate' variable can be used to configure this
  behavior.

  (org-babel-no-eval-on-ctrl-c-ctrl-c): This variable can be used to
  inhibit evaluation of code blocks with C-c C-c.

* lisp/org.el (org-ctrl-c-ctrl-c): added documentation of code block
  evaluation behavior

* lisp/babel/ob-keys.el (org-babel-key-bindings): adding keybindings
  for executing code blocks and for opening their results