multiple inputs: include source attribution

A common use case (or at least an intended use case) for multiple inputs is to have one input represent an outbreak set of sequences and then get (i) metadata colouring for this set and (ii) the ability to run proximal sampling against this set. We therefore need to know which input a particular sample came from. Currently our implementation ([at least for zika](https://github.com/nextstrain/zika/blob/main/phylogenetic/rules/merge_inputs.smk)) doesn't do this ```sh augur merge \ --metadata {params.metadata:q} \ --metadata-id-columns {params.id_field:q} \ --output-metadata {output.metadata:q} ``` I think using `--source-columns` as part of the merge is enough, but I need to test.