translate should handle variables as part of pipeline

[willkg]

dennis-cmd translate has a --varformat flag, but there are two problems with that:

1. it's ignored by most of the transforms
2. it should be like html where it breaks the token stream into mutable and immutable parts

I think instead of having it as a separate flag, we should change it so that it's passed as part of the pipeline. Something like this should work:

% dennis-cmd translate -p python-format,shouty -s "omg %(foo)s!
OMG %(foo)s!