Include basic web color settings

[ynishi]

Include commonly used color settings.

I think that basic color like The HTML defined is better to be included. https://en.wikipedia.org/wiki/Web_colors#HTML_color_names

It seems that developers often implement it myself. https://github.com/mdgriffith/style-elements/blob/master/examples/Color.elm

Or it might be better to be explained the best practice in the document.

What I did

Install and write some code with reading document. https://mdgriffith.gitbooks.io/style-elements/content/defining-a-stylesheet.html

What I Expected To Happen

• Simply include colors and use it immediately.

  import Style.Colors exposing (darkGray)
  -- ...
  Style.styleSheet [ Style.style Title [ Color.text darkGray ]]
  -- ...

What Actually Happened

• Happened error when compile sample code from document.

  
  $ elm make src/Main.elm

I cannot find a darkGray variable:

Style.styleSheet [ Style.style Title [ Color.text darkGray ]]

## Versions

- Elm: 0.19
- style-elements:5.0.1
- Browser: - (just compile)
- Operating System: Ubuntu 18.04.1 LTS