add server to serve API docs

`npm start` will serve the API on `GET /`

A BE implementation can get the express instance with `require("todomvc-api").server`, and can get just the HTML to display the API spec with `require("todomvc-api").getSpecHtml`
It's actually to serve the node_modules/github-markdown-css file. I could register a route just for that file - which is less gross?