Automatically configure Webpacker

[jamesmartin]

In order to remain backwards compatible with Sprockets and precompiled assets it was necessary to introduce the InlineSvg::WebpackAssetFinder as an "opt-in" configuration for people that use Webpack to bundle their assets.

Ideally we should be able to automatically figure out if Webpacker is bundling assets and not rely on user configuration.

This will likely be a breaking change, as it was when I tried to introduce Webpacker automatically and broke upgrading users (#97, #98).