Add assetsSubDirectory configuration param #1311
Description
What problem does this feature solve?
Some frameworks, like Flask, require assets to be in a specific folder separated from the index.html.
In #1027 (comment) there's a fix for vue-cli 3 up to beta9 but it doesn't work for beta10 and webpack 4
What does the proposed API look like?
Provide an option to configure the name of the subdirectory of dist to put assets in.