Support for export specifiers?

It appears like `workerize-loader` only supports moving [export declarations](https://github.com/developit/workerize-loader/blob/master/src/index.js#L52) in the target module into a worker. I'm attempting to use it with [Bucklescript](https://bucklescript.github.io/), which compiles modules down to an export specifier list, i.e. `export { foo, bar, }` instead of export declarations.

I took a pass at implementing this and couldn't get around Webpack [declaring its own parser plugin and bailing on `export specifier`](https://github.com/webpack/webpack/blob/master/lib/dependencies/HarmonyExportDependencyParserPlugin.js#L39), I think for tree-shaking purposes. I don't have much experience with Webpack plugins though and it seems like there should be a way to get this to work. Are there any alternate approaches you can suggest, or maybe workarounds?

One other thought I had for my purposes would be to instead compile to commonjs and tweak [workerize](https://github.com/developit/workerize) so that it would accept the pre-compiled source instead of doing the current find-and-replace compilation step.

Start with export handling in src/index.js and reproduce the BuckleScript-generated export specifier list. Read the linked Webpack HarmonyExportDependencyParserPlugin behavior to understand why export specifiers are rejected, then compare the proposed CommonJS and workerize alternatives. Done means a supported approach or a documented workaround is established and tested.

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