Fix broke image links in `extras`

Thanks @lexnederbragt for the report.

Remove the `extras/novice/` from image `src` field.

Close #295.
Fix links in `_site/book.html`

@gvwilson raised a issue that the previous commit brekas the
image links in the `_site/book.html`.

We fix this new issue with the same hack using for the other
lessons: using sed to change the links.
Merge pull request #7 from r-gaia-cs/fix-295

Fix broke image links in `extras`