Add json output for report command

[smacker]

We need it for PoC.

[smacker]

Output example:

$ ./report --output-format=json | tail -n +2 | jq
{
  "duplicates": [
    [
      {
        "repo": "github.com/erizocosmico/borges.git",
        "commit": "b1fcd3bf0ba810c05cb418babc09cc7f7783cc03",
        "path": "MAINTAINERS",
        "sha": "dc6817245a5213ab250471276acb61c186acab11"
      },
      {
        "repo": "github.com/src-d/borges.git",
        "commit": "e784f9d5f59d5c081c5f8f71b6c517918b899df0",
        "path": "MAINTAINERS",
        "sha": "dc6817245a5213ab250471276acb61c186acab11"
      }
    ],
    ...
  ],
  "similarities": [
    [
      {
        "file": {
          "repo": "github.com/src-d/borges.git",
          "commit": "e784f9d5f59d5c081c5f8f71b6c517918b899df0",
          "path": "consumer_test.go",
          "sha": "c1dc2a658b05a68f6e68cffe953d119898f2f657"
        }
      },
      {
        "file": {
          "repo": "github.com/erizocosmico/borges.git",
          "commit": "b1fcd3bf0ba810c05cb418babc09cc7f7783cc03",
          "path": "consumer_test.go",
          "sha": "f55507972a35a9fb080a41d769d7ff2526a605be"
        }
      }
    ],
    ...
  ]
}

[carlosms]

LGTM, although it would be nice to have docs for the new option. Maybe we can open an issue to document it later to not block the PoC.

[smacker]

Thanks! Added new cli params to README.