Generate archive files and links to them.

We introduce a generator that copies all the files from your `_examples`
directory into `_site`. It also creates .tgz and .zip archives of the
contents of each immediate child directory of `_examples` and puts those
in `_site`.

The `file_url`, `tar_archive_url` and `zip_archive_url` functions can be
used to construct links to these files.