bandwidth file spec

The bandwidth file spec seems very similar to a netdoc. It may even use a subset of the netdoc metaformat. It certainly should use the netdoc metaformat. Currently it has a clone-and-hack approach.

Additionally, the bandwidth-file-headers vote netdoc Item implies that each bandwidth file header Item must have only one argument, but this is not stated anywhere; the Value referenced in consensus-formats.md is not actually defined.