Add option to prefix id attribute of symbol element

[elidupuis]

Pretty self-explanatory... Lemme know what you think.

[tomwayson]

any reason not to merge this? I really need this. instead, I've had to add something like the following in order to add a prefix to all files viafileSettings:

  _createSvgStoreTree (iconFolder) {
    // prefix the file names with addon name and folder
    const srcDir = path.join(__dirname, `addon/assets/${iconFolder}`);
    const inputFiles = fs.readdirSync(srcDir);
    const fileSettings = inputFiles.reduce((acc, fileName) => {
      const key = fileName.replace(/\.svg$/, '');
      acc[key] = {
        id: `${this.name}-${iconFolder}-${key}`
      };
      return acc;
    }, {});
    // build the svgstore sprite
    return svgstore(new Funnel(srcDir), {
      outputFile: `${this.name}/assets/images/${iconFolder}.svg`,
      fileSettings,
      svgstoreOpts: {
        cleanSymbols: true
      }
    });
  },

I've had to add that to a couple addons now, and would prefer to have this just support a prefix option.

[tomwayson]

OK, I now see the issue, this PR is quite old and seems to predate the new architecture. It would now be a matter of adding the prefix here:

broccoli-svgstore/index.js

Line 73 in 9218bc8

var svgId = inputFileSettings.id || fileNameWithoutExtension;

@BrianSipple would you accept a new PR that adds a prefix option that (if passed in) will be prepended to the id at the above line?