Add uri-root option

This option allows the generated uris in production to have a prefix to their asset root. The purpose is to generate uris in production that are not served at the site root.

I wanted this feature for generating assets for a gh-pages site. Since those are served from a subdirectory, The uris generated by link-to-asset were not working since they start at site root (i.e.`/`).

Fixes #10.