Allow custom php executable path

[phaberest]

Hi!
I'm still forced to use php 5.6 at work, but I'm able to manually execute phpstan if I call it using my php7.1 executable as prefix to the phar path.

I mean something like

/usr/local/opt/php@7.1/bin/php /usr/local/bin/phpstan

Do you think it would be possible to add it to this extension?