Improve Java experience

[jeff303]

Currently, only the "value format" (string) is supported in Java mode. While full fledged compilation of dynamic functions is a big lift, it might be useful to have some type of DSL that exposes more capabilities w.r.t. mapping and reduction here.

Also, it would be good to have --value-format and --key-format as different args that get translated to the right place in xf-args. It might also be possible to have some kind of option like --output "k:str v:clj h:myHeader" (meaning, output the key, parsed as a string, then a space, then the value, as a Clojure data structure, followed by the value of header myHeader).

[jeff303]

One idea here is to use instaparse (see, ex: #17), to parse the user-designated format string. Then transform the tree via a function over each ConsumerRecord.