Not saving to file for webpack 3

[t47io]

Using webpack 3.12.0. Even with { writeFile: true } in options, no json file is written to disk. I tracked down the problem is here:
https://github.com/GoogleChromeLabs/size-plugin/blob/master/src/index.mjs#L158

For webpack 3, compiler.options.mode is undefined, which leads to no-op when writeToDisk: https://github.com/GoogleChromeLabs/size-plugin/blob/master/src/index.mjs#L118-L131

[t47io]

It seems there is no mode info at all for webpack 3 options, so one way to fix is to add optional mode override in SizePlugin(options). Ideas? @developit

[developit]

Added in 3.0.0.