implemented ability to save JSON report to file

[fumblehool]

Fixes: #257

--json flag will save json report to file.
Example $ npm-check --json=/home/user/report.json will save JSON report to /home/user/report.json

Sample report:

[
    {
        "moduleName": "babel-runtime",
        "homepage": "https://github.com/babel/babel",
        "latest": "6.26.0",
        "installed": "6.26.0",
        "isInstalled": true,
        "notInstalled": false,
        "packageWanted": "6.26.0",
        "packageJson": "^6.6.1",
        "devDependency": false,
        "mismatch": false,
        "semverValid": "6.26.0",
        "easyUpgrade": true,
        "bump": null,
        "unused": false
    }
]

[sebastianrothe]

Can this be merged, please ?

[Ephigenia]

It is pretty common to support JSON format in a CLI tool. Usually this is done with the --json option and allows additional processing to the output and also storing, or even appending to files using POSIX pipe. I’d think if this PR is changed so that it only prints out the JSON it's totally fine.

npx npm-check --json > my-report.json

And even processing would be easier, just imagine using jq for filtering and processing:

npx npm-check --json | jq '.[] | {name: .modulename}'

With just printing the JSON it would become more flexible. If you’re interested there’s a pretty good chapter about output here: https://clig.dev/#output

[Ephigenia]

Recommended change here to output JSON instead of writing to a file.

[fumblehool]

closing this PR in-favour of PR: #532 cc @Ephigenia 😄