Option to extend `transformAssetUrls`

[glen-84]

What problem does this feature solve?

If you want to customise transformAssetUrls, you have to specify all of the default values again, instead of just appending to them.

What does the proposed API look like?

I'm not sure, there are multiple options. The most obvious/simple solution would be to just export the defaults from vue-loader, which might in fact come from @vue/component-compiler-utils, so that they could be spread into user options.