feat(storybook): added controls for CSS variables

[iandi09]

Closes #453

• added css variable support to story template generator
• extracted css props for the existing stories

[janivo]

It would be cooler to extract this CSS prop from the stenci-docs.json file, so we can automatically generate the respective controls.

e.g.

/**
  * @prop --ino-avatar-size: Width and height of the ino-avatar (default:40px,size)
  */

Where the default value of 40px and the type of control (size) is extracted