[html-utils] ID fixing and fileset generation

 * html-id-fixer: add IDs to headings and sections
   to prepare for the outline generation
 * html-to-fileet: compute the resource fileset of
   an XHTML document
 * extracted px:html-load outside of the library
[html-utils] add ID de-duplication + safe IDs
[typo] svg media type
[html-utils] add an html-fixer stub
[html-utils] suppport more elements in html-to-fileset
Merge pull request #8 from daisy-consortium/html-utils

Html utils