Factor out html generation of summary files

The "hpc markup" command generates, among others, the
summary files "index_hpc.html", "index_hpc_fun.html",
"index_hpc_alt.html" and "index_hpc_exp.html". This commit
factors out the logic for generating these files into a
self-contained module, in view of using the Lucid2 templating
engine to generate these html files.