Flag to print specific JSON key

[ccakes]

This adds a --key argument which allows printing only a partial JSON object. My use case is writing logs in JSON, but when using saw I tend to filter down to just what I want and so would rather just see ${event.message} rather than the whole JSON object

saw watch /logs/nomad --prefix my_app --filter "200 OK" --key message

I don't really "know" Go, so this PR is probably pretty terrible. There's certainly no error handling. Let me know if you'd accept this functionality and I'm happy to clean it up.

[andrewpage]

I think this functionality might be best left for another tool, like jq. I subscribe to the Unix philosophy of "do one thing and do it well" in this regard.

[fquinner]

Honestly I was looking for this functionality, then I saw this MR, I didn't actually realize JQ could handle newline delimited JSON (since it wasn't a JSON list) so never thought to try, but it does work.

saw watch --raw /aws/log/group/name | egrep "^{" | jq '.message'

I needed that first curly brace grep to get around some of the logging done by aws lambda that I have no control over the format of, and raw to trim out the metadata, but otherwise works well. If it's not added to the application, I think documenting an example like this in the README could prove useful.

[TylerBrock]

Thanks @ccakes! I do agree with @fquinner and @andrewpage that this "projection" work is best left to another tool like jq but i do see some value in providing a general projection ability in saw itself (if only for performance). I might need to think about this a bit more. I'll get back to you on it.