Add support for CSS and JS liquid files

I’ve been using Vim (and vim-liquid) recently to edit Shopify theme files and I noticed that the `*.css.liquid`, `*.scss.liquid` and `*.js.liquid` files Shopify uses were being detected as Liquid files when really, they’re just normal CSS, SCSS and JavaScript files with a few Liquid tags in them.

I therefore suggest that we treat them as normal files so other plugins/settings can continue to work with them.