Multicompilation of indexXX.js files to indexXX.html outputs

Created by: plievone

Hi, consider this as a WIP / discussion opener / proof-of-concept of sorts related to #1084. With this, it is quite easy to add multiple .html outputs when needed:

• src/index.js results in index.html and accompanied bundles in build/
• src/indexXX.js results in indexXX.html and accompanied bundles in build/

Verified by copying packages/react-scripts/template/src/index.js to index2.js with modifications and seeing that both yarn start and yarn build succeeded and both http://localhost:3000/ and http://localhost:3000/index2.html work and hot-reload ok, but there may well be edge cases (such as advanced configuration and router usage) so please don't merge until polished by a knowledgeable create-react-app person.

It might be better to output pages (such as src/pages/indexname/index.js or src/pages/indexname.js) to separate subdirectories (such as build/indexname/index.html), but that may necessitate larger changes in build scripts than this minimal diff.