Polymer generator seed-element issue for .gitignore

[riteshe63]

Greetings from standard-project-structure
I am using the command yo polymer:seed my-foo to install seed-element into our project.
But .gitignore file is blank and sometimes developers push bower_components into github repo and size of the project increases.
So if we can add minimum bower_components to gitignore file, it can resolve the issue.
PolymerElements/seed-element project is defined in bower_components but generator is not defined in .gitignore file.

[robdodson]

Would you care to submit a PR for this change? You'll probably need to use an approach like this https://github.com/yeoman/generator-polymer/blob/master/app/index.js#L60-L72