fix : ignore extensions that do not have associated loader class

[ipranjal]

Thanks for this awesome library , I am using this as the configuration engine for scrawler framework https://github.com/scrawler-labs/scrawler

I have seen some shared hosting creating random weird files like .error_log inside config directory , this causes an error "Class PHLAK\Config\Loaders\Error_log" not found and causes scrawler's boot process to fail

This PR checks if the relevant loader class exists for the file type , If the loader does not exists the file is ignored and other files are successfully loaded