Add support for writing multiple reports at once

[leipert]

We are adding support for a new option called reports which accepts an
array of reports:

new BundleAnalyzerPlugin({
  analyzerMode: "static",
  reports: [
    { type: "json", filename: "my-report.json" },
    { type: "html", filename: "my-report.html" },
  ],
});

Users of the analyzerMode in json should migrate from:

new BundleAnalyzerPlugin({
  analyzerMode: "json",
  reportFilename: "my-report.json",
});

to

new BundleAnalyzerPlugin({
  analyzerMode: "static",
  reports: [{ type: "json", filename: "my-report.json" }],
});

Users of the analyzerMode with html should migrate from:

new BundleAnalyzerPlugin({
  analyzerMode: "static",
  reportFileName: "my-report.html",
});

to

new BundleAnalyzerPlugin({
  analyzerMode: "static",
  reports: [{ type: "html", filename: "my-report.html" }],
});

In order to help with the migration, these config changes will pe
printed with a logger warning, while we internally being rewritten to
the new format so that this isn't a breaking change.

[leipert]

@valscion This is code only for now, if you are satisfied with the code changes, I will update the docs.

[valscion]

I don't know if this comment by @th0r applies here: #375 (comment)

I'll let @th0r communicate here if this direction is good or not.

Hmm yeah, I think adding support for analyzerMode possibly being an array would be reasonable.

Well, I think we can do that, but it will require a change to reportFilename option to be able to return different filenames for different report types e.g. allow to make it a function (reportType) => <reportFilename> (which won't work for CLI) or an object {<reportType>: <reportFilename>}. Anyway, options becomes a mess... We definitely need to ship a new major version with completely revised API with pluggable external reporters.