Feature - Expose CSS API

[JonatanSalas]

Motivation

Currently our library is using inline styling to set styles which is really wrong because it limits our users to change the QrReader UI to match their needs.

Proposal

Implement a solution that exposes a .css file, or use a css-in-js library so the component it's encapsulated, also, add the possibility to override styles via a prop called classes like material-ui.

Eg:

<QrReader 
   classes={{ 
     root: 'your-root-class', 
     container: 'your-container-class', 
     video: 'your-video-class 
   }} 
 />