Allow filtering of meta_value in exporter

[newkind]

Hi,

There are some filters in the exporter code that allow developers to modify the exported XML data
according to their needs. One filter that is missing is the ability to filter the

meta_value

field. This can be very helpful ie. to filter out urls or any other data that's being stored there. I'm attaching a patch to be applied to the core.

[newkind]

Patch that allows filtering of meta_value

[newkind]

Changed filter name to post_meta_value

[earnjam]

We currently allow filtering the title, the content and the excerpt on export. We also allow skipping the export of individual post meta items or comment meta items, but not modifying them.

I'd be interested in hearing more about the use case for filtering the postmeta output to modify it in the export.

Also a couple of notes about the patch:
1) The name of the filter is too generic and should identify that it's used for exports, similar to the_content_export
2) The patches here in their current form cannot be applied properly. Ideally they should be created from the root of the wordpress-develop directory.