Improve and align html-output

[AlbertMietus]

I like MutMut; it has become a default tool in in my toolset. However, ...

The result is sometimes a bit hard to grasp. This while the html-report (index.html) is a very-simple table (and file does not comply to the current HTML-standard)
When doing a big project, we got a long list (table) in no particular order. So it hard to find the part (eg dir) to focus on. Nor is is conviniant to find the worst files

My request is to improve this, and align it to e.g coverage, another tool in my toolbox.

Although it's another tool, they are comparable in use and reporting. For example coverage html also gives a table with measurements pro file. See this Example Report

• One can sort (up/down) on any column (click on header)
• One can filter (see box top-right)
  Probably (read: hopefully) one can reuse a lot of that code.

Thanks

[boxed]

I personally don't use the html report and don't really see the point of it. But I am open to PRs to improve it for those who do find it useful.