Setting `HTML_PARSER_RAW_HTML = True` disables the intelligent parsing
of HTML files in flamingo.
In this mode the HTML parser uses Beautiful Soup to parse HTML conent
files. The soup is used to extract the headline of the document and to
alter the src-attributes of img-tags.

There seems to be a bug in the behavior of BS4:
https://github.com/pengutronix/flamingo/issues/15



Without this parsing any images used in HTML contents need to be copied
to the `static/`-directory instead of the `content/`-directory!

Signed-off-by: Chris Fiege <chris@tinyhost.de>