Save as HTML also exports base css

[ArnaudRinquin]

I am not quite happy with the way I extract the base css, taking it from the stylesheets having bootstrap.less as a source, which contain normalize.css and other stuff.

I'd be happy to change it if anyone has a better idea.

Fixes #232

[maxbrunsfeld]

I think it would best to avoid including all of the CSS from bootstrap.less. Ultimately, I think it'd be best to stop sharing css between the markdown preview and the rest of Atom's UI. This would also allow custom stylesheets to be used, as described in #260. I think we could put the markdown preview in its own shadow root to isolate its styling.

/cc @simurai

[simurai]

I think we could put the markdown preview in its own shadow root to isolate its styling.

That would be great. Then the markdown-preview styles can be more independant. And allowing different "markdown themes" would be neat too.

markdown-preview now uses the primer-markdown styles, which probably also depend on (or assume) normalize.css. So to fix #232, I think just picking the relevant parts from normalize.css should be fine. Like how @nickcmaynard suggested it:

.markdown-preview table {
    border-collapse: collapse;
}

Then it should also work in the future with the shadow root and when saving as HTML.