Multi mini-improvements

[Hugome]

• Option for simple mode
• Option for the main icon
  README and demo update
  Added Yarn lockfile

[hazzo]

Hi mate,
thanks for taking your time on doing a PR.
Some ideas are good but not implementable in that way.

Is it necessary to make the action button "simple" ? An html element with styles would cover that necessity. I don't think someone it's going to download the component for using it just as a simple button.

And in the other hand modifying the main icon it's a great idea but the animation has to be taken into consideration. If you change the icon now and then click the button you will get a rotated element that does not cope with the material design rules.

Please if you want to make another PR, one for each individual feature.

Thanks again!

PD: Take note that the contributors section in the package.json should not be modified. If your PR is accepted you will appear in github as one.