Adding Moar filters

Adds four filters to msm-sitemap in response to #4:

- `msm_sitemap_namespace` Filter the XML namespaces on the sitemap
- `msm_sitemap_entry` Modify the SimpleXMLElement for each entry during generation
- `msm_sitemap_skip_post` Allows skipping a sitemap entry during generation
- `msm_sitemap_query_args` Filter for modifying the query args that generates the post list