Allow for non-page-based routes

[paulsmith]

Some valuable feedback we received after our public announcement was that file-based routing is great for getting started but doesn't scale well. To a degree we are YAGNI at this early stage, but we can anticipate reasonable needs in this area for even a modest sized app.

Some of the design questions:

• Declarative vs. imperative split. Possibly the easiest first stab is just a Go file, since non-page routes will likely be Go code anyway.
• Where to put them. app/pkg/routes.go blessed path?