Fix TS types

[leebenson]

Following the Koa example, it seems some TS definitions are missing or out of sync with the lib.

Adding middleware requires any casting, such as:

app.use(webpackHotServerMiddleware(compiler, {
  createHandler: (webpackHotServerMiddleware as any).createKoaHandler,
}) as any);

... because:

1. webpackHotServerMiddleware, by default, returns an Express middleware signature, not Koa.
2. webpackHotServerMiddleware.createKoaHandler (and probably other handlers) are lacking definitions.