Error: Evaluation failed: ReferenceError: Buffer is not defined
When running `html-sketchapp --config ./config/html-sketchapp.config.js`, I now get this error:

```
npm run generate:sketchfiles

> @<project>/<project>@5.1.2 generate:sketchfiles /mnt/Workspace/<project>
> html-sketchapp --config ./config/html-sketchapp.config.js

**Error: Evaluation failed: ReferenceError: Buffer is not defined**
    at ensureBase64DataURL (<anonymous>:611:8)
    at makeImageFill (<anonymous>:631:42)
    at Style.addImageFill (<anonymous>:1464:45)
    at nodeToSketchLayers (<anonymous>:2629:25)
    at Array.from.map.node (<anonymous>:3737:13)
    at Array.map (<anonymous>)
    at getAllLayers (<anonymous>:3714:51)
    at nodes.map.node (<anonymous>:3800:21)
    at Array.map (<anonymous>)
    at Object.snapshotSymbols (<anonymous>:3796:26)
    at ExecutionContext.evaluateHandle (/mnt/Workspace/<project>/node_modules/puppeteer/lib/ExecutionContext.js:90:15)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
  -- ASYNC --
    at ExecutionContext.<anonymous> (/mnt/Workspace/<project>/node_modules/puppeteer/lib/helper.js:144:27)
    at ExecutionContext.evaluate (/mnt/Workspace/<project>/node_modules/puppeteer/lib/ExecutionContext.js:58:31)
    at ExecutionContext.<anonymous> (/mnt/Workspace/<project>/node_modules/puppeteer/lib/helper.js:145:23)
    at Frame.evaluate (/mnt/Workspace/<project>/node_modules/puppeteer/lib/FrameManager.js:439:20)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
  -- ASYNC --
    at Frame.<anonymous> (/mnt/Workspace/<project>/node_modules/puppeteer/lib/helper.js:144:27)
    at Page.evaluate (/mnt/Workspace/<project>/node_modules/puppeteer/lib/Page.js:736:43)
    at Page.<anonymous> (/mnt/Workspace/<project>/node_modules/puppeteer/lib/helper.js:145:23)
    at Object.require.config.config.usage.command [as handler] (/mnt/Workspace/<project>/node_modules/html-sketchapp-cli/bin/cli.js:146:26)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @<project>/<project>@5.1.2 generate:sketchfiles: `html-sketchapp --config ./config/html-sketchapp.config.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the @<project>/<project>@5.1.2 generate:sketchfiles script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /root/.npm/_logs/2018-11-29T09_36_43_618Z-debug.log
```

It worked before, but I can't remember if I've changed anything.

npm v6.4.1
node v8.11.1
html-sketchapp-cli v0.6.2

This is not an direct issue with `html-sketchapp-cli` I think, the error points to:

```
    if (encodingMatch) {
      buffer = Buffer.from(data, encodingMatch[1]);
    } else {
      buffer = Buffer.from(data);
    }
```

in `helpers/utils.js`: https://github.com/brainly/html-sketchapp/blob/master/html2asketch/helpers/utils.js#L78

cc @markdalgleish 