Apply proper CSS for proper page display

[benoit74]

Once #7 is implemented, we need to find a solution to apply proper CSS for proper content display.

It looks like there is only two important CSS to retrieve:

<link media="screen" type="text/css" rel="stylesheet" href="https://a.mtstatic.com/@cache/layout/anonymous.css?_=ef44df81294a27ae1883222f0b386038_Z2VvLmxpYnJldGV4dHMub3Jn:site_4038" id="mt-screen-css">

and

<link media="print" type="text/css" rel="stylesheet" href="https://a.mtstatic.com/@cache/layout/print.css?_=7cb5490395b8ed95b0d2f06424a09473:site_4038" id="mt-print-css">

[benoit74]

For the record, we need to:

• retrieve all assets referenced in a url in the CSS and store them in the ZIM
• rewrite CSS to fix all URLs, otherwise we end-up with absolute URLs to online resources and bad URLs to "nowhere"
• also fetch inline CSS (will be done in a step 2, first two points about deserve their own PR since it is already complex enough)