v1.21.0 - No longer run c-mode-hook

A minor change from the previous version, but this version has BC brake about c-mode-hook.
In past versions we needed strange hacks for co-existence with c-mode.

Added

• Add php customize group.
  • This group is positioned above php-mode.
  • This group was originally planned to be included in v1.20.0.
• Add php-mode-disable-c-mode-hook custom variable.
  • Setting this variable to nil will restore the behavior of c-mode-hook up to the previous version.

Changed

• php-mode initialization no longer executes hooks for c-mode.
  • If you want to return to the same behavior as the conventional PHP-Mode, set nil to php-mode-disable-c-mode-hook.