[Feature Request] New option base. (Aka base path, public path, asssets URL root.)

[brillout]

First off, thank you for building sirv, it's a really neat tool.

I've this use case where assets should be served from a different URL root, in my case /assets/. For example /home/brillout/my-project/publicDir/logo.svg should be served at /assets/logo.svg instead of /logo.svg.

The feature request is to be able set a new option base to the value /assets/ making sirv serve all assets at /assets/*.