Extend inlining to handle raw files (files with unrecognized extensions).

Example usage:
  [[!inline pagenames="somefile.txt" template="raw" feeds="no"]]

Also raise an error in IkiWiki::pagetype($file) if $file is blank,
which avoids trying to do much with missing files, etc.
Document raw.tmpl in doc/templates.mdwn.
Encode <, >, and & as entities during raw inlines.
Give full namespace for HTML::Entities::encode_entities().