Add author to manifest.json

This is an optional field as defined in the [manifest docs][]. I'm not
sure whether it's ever displayed anywhere but I don't think it can hurt
to set it.

[manifest docs]: https://developer.chrome.com/extensions/manifest
Add description to manifest.json

As recommended by the [manifest docs][]. This shows up in
chrome://extensions and, I presume, in the Web Store when it's uploaded.

[manifest docs]: https://developer.chrome.com/extensions/manifest