Enable to customize devServer's historyApiFallback option

[ghostrick]

• [x] I ran npm test for the package I'm modifying

  I did it, but some errors occurred. I do not think that the cause of this is mine.

• [x] I used npm run commit instead of git commit

  I can not find this command.

• [x] I have rebased my branch onto master before merging

  I did it.

This is necessary when using the router library such as cyclic-router.

$ npm start -- --enable-fallback or $ ENABLE_FALLBACK=true npm start

Please let me know if you find my mistake. I am still immature, but I am very interested in the activities of the Cycle.js community.

[jvanbruegge]

@Ghistrick If you want customization of the flavor without eject, I suggest you use the one-fits-all flavor. There you have full control over the weboack config

[ghostrick]

@jvanbruegge ok!! Thank you!!!