checks that the user wants c++
fixed whitespace error, tried to clean up names
remembered some list sugar, stripped a println
omg it's magic
supports the api kenton suggested
Merge branch 'check-language' of https://github.com/mosesn/capnproto

Supports choosing output languages on the command line.
Flesh out capnpc command-line interface.
Update docs for new capnpc command line.