"max-blob-size" config var to limit generated HTML size

Sometimes it is not feasible to generate the HTML pretty-print for large
files, especially if a source-filter is involved or binary data is to be
displayed. The "max-blob-size" config var allows to disable HTML output
for blobs bigger than X KBytes. Plain downloads are not affected.

Signed-off-by: Georg Lukas <georg@op-co.de>