"oneline" format that puts extra fields on same line

[trentm]

See #15 (comment)

Something like:

$ cat some.log | ecslog -i foo,bar
[2021-02-11T06:24:53.251Z]  WARN (myapi on purple.local): something went wrong
    foo: 1
    bar: 2
...
$ cat some.log | ecslog -i foo,bar -f oneline
[2021-02-11T06:24:53.251Z]  WARN (myapi on purple.local): something went wrong (foo:1 bar:2)
...