Minify CSS and JS

This requred moving the Javascript search code to a separate files. And,
since I only want to load it in the search page, I also added an
extra_css and extra_js strings to the header template so that I can
easily load additional CSS or JS files on specific pages.
Show and search date on search page, minify js/css with uglify

Now we minify the resulting built website so we can treat those files
as you normally would in a Makefile, only rebuiling if changed (which
happens to be every time you run jekyll).

Also, unrelated, display the created date on the search page and allow
searching for items by a specific year.