Main: Add README file
Main: XSLFactory.lookup should return nil

Return nil instead of raising exception when no xslt is found in url-xsl mapping
loaded into factory instance.
Main: Extract transform and media download into class

Move code handling HTML transformation and media file(s) downloading into
separate class.
Main: Change application command line parsing

Add --directory command line switch for changing output directory from '.' to
arbitrary one. All resource links in generated HTML will be relative to this
directory.

Remove explicit output file and directory name and allow supplying multiple urls
on the command line. Filenames are generated from position of the url on command
line, media directory name is fixed.