Add the option to choose which .html to create

[vv-monsalve]

In the old gftools gen-html there was the option to choose only one of the possible .HTML results (either Glyphs, Waterfall, or Text) which was very useful when in need to perform only one specific test while making the process faster.

It would be really useful to have a similar option in diffenator2, both at proofing and diffing time.