emit parsing-friendly output

[chenglou]

We'd like to make custom reporters for Jenga to prettify some output. It'd be nice if there was a mode to stream out e.g. line-based JSON, for other tools to consume.

[v-gb]

Have you considered using https://github.com/janestreet/jenga/blob/master/lib/rpc_intf.ml#L31, in combination with https://github.com/janestreet/jenga/blob/master/lib/jenga_client.mli#L23 ?

[v-gb]

Since you're probably not familiar with async-rpc, here is an example: https://github.com/janestreet/jenga/blob/master/lib/cmd_watch.ml#L49-L86, and you can run with [jenga errors].