Allow --tag specification

[BenLangmead]

Since people may do multiple large, expensive runs with Rail, like we do, it's nice to be able to tag the EMR cluster so that the various projects can be distinguished on the bill.

[BenLangmead]

Maybe you have this already? I see "Tags": [], in the output JSON, but there are function in the code that have tags='' in the parameter list, so I assumed it wasn't working.