Add a Files trait

[stevegrunwell]

Some platforms require files to be put into-place (WordPress' wp-includes/version.php comes to mind), so it'll be helpful to have a trait that will temporarily move old files aside and symlink in new alternatives.

Potential usage:

$this->replaceFile(ABSPATH . '/wp-includes/version.php', STUBS_DIR . '/version.php');

Old files should never actually be deleted, so maybe they're moved aside with a .bak extension (or similar).