Suffix the archiveDir only if non-empty

If `archive_dir` equals "archives" in _config.yml, the output will be: `/archives/2013/`

If `archive_dir` equals "" in _config.yml, the output will be: `//2013/` (which leads to bad interpretation by the browser; expecting `/2013/`)

Also avoids computing the base URL for each link (either it is yearly or monthly).