Standard color palette and colors review

[afercia]

Fixes #106

Just a first pass for now, still have to create a folder where to put things (maybe create a README.md) and a colors.json but need to share the current progress to discuss a few things. I guess conflicts can be fixed later.

Things that need a decision/feedback:

• naming conventions
• underscore or hyphen? 😬
• what to do about rgba greys (mainly box-shadows)
• should the color schemes be included in the main colors file or be separated in their own file?

The base idea is to establish a general, standard, colors palette that can be used everywhere and then re-map the currently used colors to the palette-colors.

Platform-specific colors (e.g. WordPress specific link colors) shouldn't be part of the palette.

Worth noting I'm updating stuff also on YoastSEO.js on a local branch.

[afercia]

After some research, I'm trying grunt-scss-to-json to generate the colors.json file. See details:
https://www.npmjs.com/package/grunt-scss-to-json
https://github.com/ryanbahniuk/grunt-scss-to-json

There are more advanced tools (also a ruby gem and couple of JS modules) also for the other way around (JSON -> scss) but this one is simple and seems to work for our needs.
I'd defer to @atimmer for the final decision :)

[afercia]

Tested and re-tested, there are things that can be improved of course but I think it's time for a first code review and feedback :)

Side note, there are a few things to document, should they go in the readme file?

[andizer]

CR Done 👍

[terw-dan]

Acceptance 👍