Add a build task to the Gruntfile.
Add a build task to the Gruntfile.

Instead of running `cfx xpi`, the user can directly run `grunt build` and collect the `.xpi` file from the `dist` folder.
gutsal-arsen/generator-firefox-extension,dgil/generator-firefox-extension,gutsal-arsen/generator-firefox-extension,dgil/generator-firefox-extension