Fix spinner output to stderr

[kawaz]

Before

There was a problem when handling raw logs with pipes. Because the output of spinner was stdout.

$ utern --no-log-group --no-log-stream RDS | jq .
+ RDSOSMetrics
+ RDSOSMetrics › db-T7X6VABCCNHPRQEROUSDIRSTUU (2019-11-22T15:02:47+09:00)
parse error: Invalid numeric literal at line 1, column 4

After

So I changed the output of spinner to stderr.

$ ./utern --no-log-group --no-log-stream RDS | jq .
+ RDSOSMetrics
+ RDSOSMetrics › db-T7X6VABCCNHPRQEROUSDIRSTUU (2019-11-22T15:03:47+09:00)
{
  "engine": "MYSQL",
  "instanceID": "sugoi-db",
  "instanceResourceID": "db-T7X6VABCCNHPRQEROUSDIRSTUU",
  "timestamp": "2019-11-22T05:59:47Z",
  "version": 1,
  "uptime": "83 days, 07:20:09",
  "numVCPUs": 2,
  "cpuUtilization": {
    "guest": 0,
    "irq": 0,

[Songmu]

I've confirmed it to work with the following command.

% utern awslogs-api 2> /dev/null

Thanks!

[Songmu]

Released as v0.0.4.