Improve Previews

[poef]

Previews aren't always completely up to date, you need to save a component to see all CSS changes reflected. This should not be necessary. In addition, performance of previews gets worse the longer you use SimplyCode.

• Create an improved preview builder, which uses all uptodate code
• Only build previews if they are viewable (in a viewable area in the browser)
• Allow a preview to be opened in a seperate tab/window, keep live updates going

[ylebre]

The performance issue has already been addressed previously, this should no longer be an issue.

This PR: #23 adds routes to each preview so that they can be viewed separately.

Live updates to the previews are a bit more complicated, I think these warrant a phased approach:

1. include changes in the CSS from the current component.
2. sync changes from anywhere, including from different pages. This is needed to be able to update previews in separate windows, as well as syncing changes from other components (like a base style change). We can build on localStorage for this.
3. allow the sharing of the 'state' using something like hyperhyperspace, so we can even live update across devices.