Make absolute paths work with base option

[nobkd]

base option needs some love for JS-imports and CSS-url()s and -imports.
Currently, when building, you need to ensure, that your imports are relative.
This might break, if you have more pages e.g. in subdirectories.

I'm not really sure, how this should be handled, because sometimes you really want it to be an absolute path, even when building for a subdirectory...

[tipiirai]

I think we should ditch base, since it's done so left-handed. Need to make it solid later if we plan to support it. Thoughts?