[Feature Request] --pretty option

[pxwee5]

So that we have something like this

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  </symbol>

  <symbol id="icon-refresh-btn" viewBox="0 0 100 100">
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  </symbol>

  <symbol id="icon-angle-up" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
    <path d="M75,49.9L50.1,25L25,50L0,75h16.6l16.7-16.7l16.8-16.8l16.8,16.8L83.6,75l8.2-0.1l8.2-0.1C100,74.8,75,49.9,75,49.9z"/>
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