fetch: add json output option which includes source span data

[seaneagan]

Similar to https://github.com/alanshaw/david/issues/47

den fetch --json

{
  "foo": {
    "status": {
      "current": ">=0.10.0 <0.11.0",
      "primary": "1.0.0",
      "latest": "1.1.0-0",
      "updated": ">=1.0.0 <2.0.0"
  },
  "valueSpan": {
    "start": {
      "line": 10,
      "column": 9,
      "offset": 172
    },
    "end": {
      "line": 10,
      "column": 19,
      "offset": 182
    }
}

This would allow for editor integrations.