Allow hidden CSS src in tabbed examples

[NiedziolkaMichal]

Currently when we want to add new font to single HTML interactive example, following steps must be taken:

1. Add font to live-examples/fonts directory in interactive-examples repo
2. Add the same font to live-examples/fonts directory in BOB repo
3. Add @font-face to shadow-fonts.css in BOB repo

This PR is meant to simplify this process, but allowing custom fonts to live only in interactive-examples repo and assign them to individual examples without showing @font-face to the user. It does that by introducing cssHiddenSrc property in meta.json in tabbed examples. Now process of adding new font will look like this:

1. Add font to live-examples/fonts directory
2. Add CSS file with @font-face anywhere in interactive-examples repo, but preferably near font
3. Add cssHiddenSrc property to meta.json of an example, containing path to CSS.

Another benefit of this solution, is that @font-face will be added only to examples that actually use them, while now shadow-fonts.css has to be downloaded for every example.

If this gets merged, I will add CSS files with individual fonts to interactive-examples repo, so shadow-fonts.css can be completely removed from here, along with the fonts.

Fixes #822

[queengooborg]

Looks good to me!

[NiedziolkaMichal]

Great, thanks!