Add option to generate minified stylesheets

[tannerdolby]

Description

This should be a step in the right direction. I left comments in the minify.js source to address things in code review and then we can remove most of the comments when this is closer to being "finished".

Why is this important?

This allows users to take advantage of the build:minify command to generated minified versions of each stylesheet.

Covered test cases

Haven't written any tests for this yet

Did you test on all major browsers?

• Chrome
• Firefox
• Edge
• Safari

I apologize for including the package-lock.json in the first commit, it has a large diff (-6k) so I'm happy to remove that commit if you need.

Well I tried using git rm package-lock.json with git commit -a -m "msg" but messed things up, unless I do a rebase the 6k diff from package-lock.json in the first commit will stay. I'm completely okay with creating a fresh PR that doesn't include a committed package-lock.json to avoid such a large diff, just let me know.

T&Cs

• I confirm I have read and understand the contributing guidelines
• I understand the work in this pull request will not be released straight away and will appear in a future release (if approved)
• I confirm the work in this pull request is true and valid to the best of my knowledge
• I have updated the README, features.md and codes.md files where applicable