Support external tools to supply fixups

[hexmode]

PHP CodeSniffer (phpcs) is integrated with FlyCheck. Some of the issues it finds can be fixed with phpcbf. It would be nice if this tool could get the fix needed at point from phpcbf.

https://github.com/squizlabs/PHP_CodeSniffer/wiki/Fixing-Errors-Automatically

[jyp]

That would be great indeed. Note that there is nothing preventing using an external tool. Because I never use PHP I'll wait for a contribution regarding the tool in question.