Custom meta key

I ran into trouble trying to integrate with two existing WP projects already using the 'attachments' post meta key and urgently needed a way to filter the key.

I thought an implementation of `apply_filters` would be nice, but since there's already the approach of using constants for general options (vs per instance), I went with constants instead.

So, for example:

`define('ATTACHMENTS_META_KEY', '_custom_meta_key');`

The added benefit is being able to use an underscore-prefixed meta key, so it doesn't show up in the built-in custom fields meta box. I figured version 3 and above moved away from `_attachments` for legacy reasons.