Introduce php-extra-constants

This patch provides a new customization variable:
php-extra-constants.  It is a list of strings that PHP mode will treat
as additional constants.  By default it is an empty list, but
programmers may find it useful to add constants from frameworks they
routinely use, for example.

Signed-off-by: Eric James Michael Ritz <lobbyjones@gmail.com>
Add François-Xavier Bois to the list of contributors
Describe php-extra-constants in the README
Merge branch 'fx/declare-new-constants'

* fx/declare-new-constants:
  Describe php-extra-constants in the README
  Add François-Xavier Bois to the list of contributors
  Introduce php-extra-constants