initial commit of wxr export for vanilla WP

This is pretty straight forward. The WXR export file in its current state imports to other PB installations only. This export routine extends the WXR class in order to modify the xml so that it can be consumed by a vanilla installation of WP. 
* it changes the labelling of the current WXR routine from 'WordPress XML' to 'PressBooks XML'
* it labels the vanilla WXR export routine as 'WordPress XML'

The manipulation of the xml file can be summarized by:
* replacing custom `<wp:post_type>` from 'chapter', 'front-matter', 'back-matter', 'part' to 'post'
* replacing `<category domain="back-matter-type" nicename="appendix">` and all PB variations of those to `<category domain="category" nicename="uncategorized">`
* removing any `<wp:term>` declarations

On the export page, it assigns the class attribute `vanillawxr` to any file with `_vanilla.xml` in its filename and the css has been modified to look for a .png called `vanillawxr-36.png` and `vanillawxr-48.png`. 

Creating these files is something that has been intentionally omitted from this PR
* I know you have source files for the current file icons and 
* I can imagine PB would prefer to control the look and feel of it. 

I'm not 100% sure of the need for `libxml_disable_entity_loader` in this particular case, since we're loading something we're creating,  but I've put it in anyways — please chime in if you think we can live without...