Consider running tests on Windows

The company I work for uses Windows, and we've been using ember-cli-addon-docs to document an internal addon. It is pretty evident that this code-base has not been run on Windows by the developers before. This is fair, but ultimately problematic. Ember-cli-addon-docs is increasingly used by addons across the ecosystem, essentially locking Windows users out of contributing to their documentation unless they are familiar with common issues and can debug/fix them.

We got it to work for us (although there are still rough edges), but I think this is something that will continue to break if Windows support is not part of every PR review. For this reason I propose running the test suite on Windows as well, for example using Appveyor. This is free for open source projects, just like Travis.

Please let me know what you think.