Miscellanous corrections with very minor impact

Hello, while tracking down a bug which in the end happened to be totally unrelated to `gulp-webpack`, I checked the source and made some very minor modifications. Feel free to reject if you don't approve :)

- Corrected several syntaxical inconsistencies (quoting, single-statement-
  blocks, indentation, `opt = opt || default` idiom), no semantic impact. (If instead you wanted to check specifically for `undefined`, the previous versions wouldn't work either)
- Added `.editorconfig`
- Default value for `stats` in the `done` callback (when webpack crashes
  before generating `stats`, eg. file error)
- More idiomatic `PluginError` construction (`Error` object `err` is passed instead of `err.message`)