Create example using postcss

[hursey013]

Changes proposed in this pull request:

• Removed dependencies execjs, autoprefixer-rails, jekyll-assets, from Gemfile
• Added postcss and postcss plugins to compile sass and autoprefix
• Added node_modules plugin to Jekyll to allow easier referencing/copying of assets from uswds npm package
• Adjusted {% asset %} tags to use native html tags

Security considerations

Removes unmaintained dependencies.

[hursey013]

Not totally sure what to do with this... maybe just leave it as a branch for now until we revisit the future of Jekyll starter templates?

[drewbo]

Does anything prevent us from using this as the new base template? If we think it's more reliable/maintainable without jekyll-assets it would be a good replacement for now

[hursey013]

Does anything prevent us from using this as the new base template? If we think it's more reliable/maintainable without jekyll-assets it would be a good replacement for now

It could be, however it seems like USWDS 3 continues to recommend using gulp, so I'm not sure if we want to follow that approach, or deviate from it with PostCSS (or another build tool, webpack?). Either way, probably worth talking about it more before making a switch so we can talk about whats most sustainable.