Add support for php-cs-fixer (⌃⇧H)

It is possible to set custom options via TM_PHP_CS_FIXER_OPTIONS (e.g. -vvv).

Installing ‘php-cs-fixer’ via homebrew requires setting ‘phar.readonly = Off’ in /usr/local/etc/php/«version»/php.ini. It also depends on homebrew’s php, which requires running ‘xcode-select --install’ (see issue Homebrew/homebrew-php#1212).

Closes #25.