htmlcopy a source HTML file into DESTINATION. If the HTML file have links to images, other HTML files,
javascripts and cascading style sheets, htmlcopy changing link path in the HTML file to keep the link
destination.
When DESTINATION is omitted, the modified HTML is written in the standard output. Also it is assumed that
output location is the current working directory.
SOURCE and DESTINATION should be cleanuped pathes. For example, a verbose path like '/aa/bb/../cc' may
cause converting links wrongly. This is a limitaion of the URI module's rel method. To cleanup pathes,
Cwd::realpath is useful.