Add sos-report tpl

[gaow]

Currently we've customized the tpl files so the HTML generated by using sos ... --template sos-report directly does not fit in (no TOC, header etc) But it should be simple enough (though may time some time, therefore later ... ) to adapt sos-report style here. Maybe to trigger it we need to both see kernel: SoS in the notebook AND have a configuration option for it.

[BoPeng]

There could be many such templates, with/without TOC, and with customized logo, style etc. In the end this is all Jinja templates that can be modified in whatever way users desire.

[gaow]

Currently what we have are:

1. with/without TOC
2. remove / add the logo on the right side
3. partial/full support all bootstrap themes.

There are often a "style" part and a "core" part of a template. I'd only take the "core" part of sos-report so that it can easily play nicely with those styles, or in fact, any style, if we decide to provide.

[gaow]

I simply copy-paste contents of sos-report template to my template. It works, and I make the behavior default.

But maybe we can improve the floating window? For example it kind of conflicts with the sidebar TOC and the website banner header.

In Rstudio notebook output to HTML there is a button one can click to control for cell-level folding. eg:

This looks nicer ... not sure how hard it can be implemented.