Fix paths to additional CSS and JavaScript files

[wirelyre]

Expressions in an #each block need to begin with ../ to reference values in the main context.

Closes #774.

[mattico]

Are the additional-css/additional-js resources not being copied into the book root on build time? If so I think that would be the correct fix. I don't think we generally want to have URLs referencing paths above the book root.

[wirelyre]

No, they are. This PR just fixes references to the build root.

Previously, this line produced <script type="text/javascript" src="some-file.js"></script> irrespective of the directory depth of the generated file. Now it produces, for example, <script type="text/javascript" src="../../some-file.js"></script> when generating two/subdirs/file.html.

(This is the same directory level as the auto-copied book.js and favicon.png.)

I thought that there was a different underlying problem too, but it turns out that {{#each}} blocks just need dots due to namespace rules.

[mattico]

Ah, okay. I was wondering why the ../ was inside the template. I'll try this out locally.

[mattico]

Thanks! I think ideally we'd have a test for this, but I won't make you write all that scaffolding.