Prefetch/preload any Webpack asset instead of HTMLWebpackPlugin tags

Right now the plugin will only add prefetch/preload `` for assets, that already have a tag somewhere else in the document (where the tag was generated by HTMLWebpackPlugin).

For my usecase I'd like to be able to prefetch assets that are loaded using Webpack's file-loader. My application is loading an image and a font using file-loader and referencing the files dynamically in the bundle. The paths to the assets don't appear in the HTML template, but do appear in the bundle. Right now they are just not picked up by this plugin.

I'd like to be able to apply the filters (e.g `img/*.png`) here, so that prefetch links for the images are added to the template. This seems to echo what @chrisui's mentions in [his comment](https://github.com/jantimon/resource-hints-webpack-plugin/issues/4#issuecomment-310683495).

Start by tracing how the plugin identifies assets already represented by HTMLWebpackPlugin tags, then inspect how its filters could apply to assets referenced in the bundle. The change is complete when matching file-loader image and font assets produce prefetch or preload links in the generated HTML template without requiring existing tags.

Written by the indexing model from the issue text.