Give ability to configure setting more data on pages

I've added the ability to specify in the config file additional data that will be gathered from the individual pages and made available to the theme. The config value is an array in the same format as the `$headers` array in the `read_file_meta()` method in lib/pico.php.

I am requesting this be added, as it will make a tagging plugin that I have written possible. Additionally (and more importantly), it will give a lot more options to all sites if developers want to store other "meta"-type values along with pages.