Introduce cat-thrift command

> would be possible to do this with just one mapper and passing on the format.

It would. I wouldn't say it was entirely conscious, although I was originally thinking/planning on  having multiple inputs like snapshot outputs (eg. `-i sparse=path1 -i dense=path2`) until I started worrying how to deal with the split output.

In any case I think I'm tempted to leave it as it is unless you prefer I switch it over.