support webpack multiple configurations

[re-fort]

support when webpack config is exported multiple configurations.
it might be nice to select a configuration which one to use, but this PR does not add this option, it always uses the first one.

[re-fort]

@mrjoelkemp could you review this PR?

[mrjoelkemp]

Thanks for this @re-fort. So that I understand this is fixing a bug where a multi-config causes this lib to break?

[mrjoelkemp]

Is it possible to have webpack choose the right config instead of doing that here by defaulting to the first one?

[re-fort]

So that I understand this is fixing a bug where a multi-config causes this lib to break?

Yes, that's right.
when loading multiple webpack configurations, this lib doesn't work.

Is it possible to have webpack choose the right config instead of doing that here by defaulting to the first one?

it would be not able to do that, the right config needs to be selected by the user.
the solution to let the user choose which config to use:

1. add an option to webpackConfig like

webpackConfig: {
  path: 'path/to/webpack/config',
  option: {
    webpackConfigIndex: 0
  }
}

if doing this, need to consider backward compatibility.

2. create a new webpack config file for this lib like

const config = require('path/to/webpack/config')[0];
module.exports = config;

it's workaround and no need to change this lib.
it's a little weird for me that having the webpack config only for this library, so I made this PR.

[mrjoelkemp]

Thank you @re-fort. Appreciate the fix with a test. Thanks for contributing. Pushed this as a new patch version.