Support Webpacker

[jamesmartin]

Raised in #95 and #91. inline_svg currently relies either on Sprockets or a naive static asset finder that just looks for assets in the configured assets directory.

This branch enables the gem to be automatically configured with a new kind of asset finder, WebpackAssetFinder, which uses the Webpacker gem to locate the SVGs on disk.

Note: To make this work automatically it's necessary to completely disable Sprockets in your Rails app. If you can't do that for some reason, then manually configure the WebpackAssetFinder like so:

InlineSvg.configure do |config|
  config.asset_finder = InlineSvg::WebpackAssetFinder
end

[jamesmartin]

I found the following things helpful when making this work:

• 👋 Goodbye Sprockets. Welcome Webpacker
• From the Asset Pipeline to Webpack
• The Rails Webpacker gem source code

[jamesmartin]

Released in v1.5.0