User customizable fill-paragraph behavior.

User customizable fill-paragraph behavior.

For this, four new variables which contain the symbol name of the
function that specifies the behavior of fill-paragraph on certain
conditions were added:

    * python-fill-comment-function: For comments
    * python-fill-string-function: For strings
    * python-fill-decorator-function: For decorators
    * python-fill-paren-function: For parens

All of these variables are safe local variables in the case the value
provided is a symbol. Out of the box, they default to these four new
functions respectively:

    * python-fill-comment
    * python-fill-string
    * python-fill-decorator
    * python-fill-paren