Add lisp-fill-string-by-source-column

[melisgl]

With lisp-fill-string-by-source-column nil (the previous default
behaviour):

(defgeneric foo ()
  (:documentation "The first line of this indented docstring is roughly as long as
the rest. Using this option makes sense to ensure that docstrings are
preformatted."))

With lisp-fill-string-by-source-column t:

(defgeneric foo ()
  (:documentation "The line of this indented docstring is as
  roughly as long as long as rest. Use this option to make source
  code look nice."))