HTML output too large

[phiwuu]

The HTML output is too large. If 800 items are added to 3 levels, then users complain about long loading times.

See also #38. I wonder if the HTML report is the correct approach for large reports (>200 items).

[phiwuu]

The long loading time in Google Chrome comes from the many SVG images. Maybe there is a way to render the images only once, and re-use them (like this)