fontify most magit `with' and `define' macros

In libraries that are part of Magit automatically add font-lock
keywords for macros like `magit-define-command' and
`magit-with-section'.  This can be turned off by setting
`magit-add-font-lock-keywords' to nil.
do not activate magit specific font-lock keywords

We haven't decided yet how to best activate these keywords.
So for now users have to activate manually in all elisp files:

    (font-lock-add-keywords 'emacs-lisp-mode
                            magit-font-lock-keywords))
Merge branch 'font-lock-keywords' into next