add prefix-stripping (like -prefix /XYZ)
add prefix-stripping (like -prefix /XYZ)

Now you can run `toolbag -prefix /XYZ' to remove `/XYZ' from paths
before ToolBag's ServeMux processes them. Because I've only wrapped the
ToolBag in a StripPrefix, this still leaves plenty of links horribly
broken, namely the ones generated by the Tools tool. At the moment, it
prints aTool.Path(), an absolute path, without any knowledge of the
stripped prefix.

Still, this change makes testing easier.

ausbin/toolbag,ausbin/toolbag