Automatic creation of guides listing on main page.

JSDuck creates HTML for the guides section, listing all the
available guides specified with --guides option, and places
the HTML in index.html inside a hidden div.

Tweaked styles to automatically adjust to fit all the guides links
to the Guides section at index page.  Previously the column-widths
were simply hard-coded.

Additionally the guides section will now be completely turned off
when no --guides option specified.