Reorganize shared folder and update guide for authors

[nokome]

This attempts to finally close #25 with a reorganization of the src/shared repo and some guidelines and examples for theme authors. It will add two new themes

• bootstrap: an example of bootstrapping a theme from an existing stylesheet (as with elife theme)
• zombie: an example of a theme that relies heavily on the styles defined in shared (cf. skeleton which does not at all)

@davidcmoulton @alex-ketch This is still WIP but I wanted to make you aware of it. Especially the use of the shared/js/dom functions to modify the DOM as needed to apply an existing stylesheet: https://github.com/stencila/thema/pull/52/files#diff-4b8a23a18d6432149ae65d5eca083717R1

The to do list for this PR:

• Document more fully when and how to use index.{js|ts} and functions
• Get npm run lint:styles to pass
• Flesh out the styles in src/shared/styles/types and use them in zombie

[nokome]

After some discussion on the pros and cons of alternative approaches, @alex-ketch and I agreed that it would be best not to have shared/styles. Instead, we will "flesh out" skeleton (maybe it needs a different name) so that (a) it has some reasonable defaults and is not just empty placeholders and (b) that many / most of those defaults will be variables that will allow that theme to be easily extended as new themes. The related shared/mixins can also be removed.

[stencila-ci]

🎉 This PR is included in version 1.6.0 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release
• v1.6.0

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