Allow users to override default template in InterpolateHtmlPlugin

Created by: gautier-lefebvre

This PR intends to allow users to override the default %VAR% template in index.html. In production, my company uses a nginx which changes variables in the $VAR format at runtime (using Docker, when starting the container), so I find it easier to keep my variables in the same format for webpack in development mode via webpack-dev-server.

Testing

1. Forked the repo at https://github.com/lfbvr/create-react-app.
2. Run npm link in that repo.
3. Run npm link react-dev-utils in my project folder.
4. Run webpack with this config:

webpack.config.js

index.html (template)

index.html (result)

Setting the InterpolateHtmlPlugin without options works as before.