SVGs not being imported
Hi all,

Noticed the latest work done by @kdzwinel @KimDal-hyeong on adding SVG support (huge thanks, guys!) and had a go using the latest code (actually I was doing that _while_ MR #47  was being put forward lol).
However, it's still not working for me. Code below:

```
<section>
  <svg class="sg-icon" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16"> 
    <g>    
      <polygon fill="#000000" points="8,0 16,8 16,16 10.1,16 10.1,12 5.9,12 5.9,16 0,16 0,8 "/>
    </g>
  </svg>
</section>
```

Build & inject go through without errors. Console throws no errors either.

What I end up after importing in Sketch is an empty 16x16 sg-icon Symbol. Empty as in the symbol has no layers or groups inside of it. Sure, the size of the symbol is correct (16x16) but I can't figure out why the actual vector isn't being imported inside of the symbol.

Happy to try any ideas you throw at me.