Add --host option

[tanaka51]

refs #176

I added --host option to start command.

The rackup command runs in the back of start command binds localhost as default on rack/rack#514 . This option enables to change this.

edited: I changed default host to 0.0.0.0 from localhost for backward compatibility.

$ ./bin/fluentd-ui start
Puma 2.11.1 starting...
* Min threads: 0, max threads: 16
* Environment: production
* Listening on tcp://0.0.0.0:9292

$ ./bin/fluentd-ui start --host localhost
Puma 2.11.1 starting...
* Min threads: 0, max threads: 16
* Environment: production
* Listening on tcp://localhost:9292

[uu59]

default should be 0.0.0.0 for compatibility.

[yoshihara]

LGTM