WordPress: Only ignore files in root to support other plugins

Most files are ignores without defining the directory.
This caused trouble with several plugins.
To fix that some files should only be ignored in the root directory.

- .htaccess is only ignored in the root directory
  due to some plugins or devs that need and create the htaccess
  for security issues in subfolders
- sitemap.xml and sitemap.xml.gz is only ignored in the root directory
  due to plugins that have an equal name and are therefor ignored
- Those files are ordered alphabetically