Conform settings to metabook.json spec

 * Renames showtoc to toc

 * Sets the default columns to 2

 * Adds auto as an option for the toc

 * Fixes handling checkboxes in js

 * Puts the settings directly on the metabook object

Bug: 68836
Change-Id: I2e0941925284d32756118d01043f5215ddd51401