Add DisableEscapeHTML parameter to JSONFormatter

[Dakatan]

Hi!

I use logrus, but http request and response xml is escaped by json encoder.
So, i add this parameter.

Logging sample.

1. DisableEscapeHTML=false or default

{"level":"panic","msg":"","time":"0001-01-01T00:00:00Z","xml":"\u003cxml\u003e\u0026\u003c/xml\u003e"}

2. DisableEscapeHTML=true

{"level":"panic","msg":"","time":"0001-01-01T00:00:00Z","xml":"<xml>&</xml>"}

Thank you!

[markphelps]

Fixed in #1113. Thanks!