Single-page app easy routing

Inspired by [this branch on carrot/charge](https://github.com/carrot/charge/tree/spa), this implements an `spa` configuration option which simply manipulates the configuration before the middleware chain is instantiated.  This is entirely for convenience.

Setting `spa` to `true` manipulates `clean_urls` and `custom_routes` to install a routing template for single-page applications:
- It forces `clean_urls` to `true`, and.
- It extends `routes` to include the following routes:

  ```javascript
  {
    "./!(*.*)": "/index.html",
    "**/!(*.*)": "/index.html",
    "./*.html": "/index.html",
    "**/*.html": "/index.html"
  }
  ```