Add background change to stories and preview-head

[guastallaigor]

This PR adds background option to storybook and a preview-head to center the components, for a better development in my opinion.

With @storybook/vue I didn't manage to import the background globally, so I had to import in each of the components. If you know a way to import globally, please let me know.

Thank you.

[codecov]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #6   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines          26     26           
  Branches        2      2           
=====================================
  Hits           26     26

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[fernyettheplant]

This is a great improvement for Storybook

the only thing that I noticed is that the yarn.lock was deleted... can you generate it again by doing yarn install please?