Emitters: add output: file

[hifly81]

GLOBAL Section:

{
"global": {
"seed": -1,
"templateDir": "$HOME/.jr/templates",
"output": file
"kafkaConfig": "./kafka/config.properties
....

If specified in the global section, additional configs (example kafkaConfig) will not be considered.

Entry section:

{
"emitters": [
{
"name": "shoe",
"locale": "us",
"num": 0,
"frequency": "0s",
"duration": "0s",
"preload": 10,
"valueTemplate": "shoe",
"output": "file",
...

if not specified in the global section, every emitter can add its own file or different output.