Test path interpolation with relative URIs

It should work the same as if a user typed the full path where the
variable goes.  Just like interpolation should always work.
Interpolate path terms like a/b/c.html

Instead of specifying a path with 'a/b/c.html' one can now specify the
term a/b/c.html (with SWI V7).  This is often more natural than using an
atom.  It also allows some of those path components to be variables.
Release v0.3.0