allow WP_Parser\get_wp_files() to follow symlinks

[pbiron]

For those of us using this plugin outside of the .org Code Reference, it would be really helpful if WP_Parser\get_wp_files() could follow symlinks it encounters while recursing the directory structure.

All that would be necessary to enable this would be to instantiate the RecursiveDirectoryIterator with:

new \RecursiveDirectoryIterator( $directory, \FilesystemIterator::FOLLOW_SYMLINKS )

instead of the current:

new \RecursiveDirectoryIterator( $directory )

[pbiron]

I can do a PR, but I think that #208 should be merged first.