Pages plugin: file-based routing

The built-in pages build plugin scans a directory (default pages/) and
generates route registrations from its structure, so the file tree is the
route table:

pages/
  index.go        // -> /
  about.go        // -> /about
  posts/[id].go   // -> /posts/:id

How it works

Before every build the plugin walks the pages directory, derives a route
for each .go file (test and generated files are skipped) and writes a
temporary routes_gen.go in the same package:

// Code generated by pages plugin. DO NOT EDIT.
func init() {
    router.RegisterRoute(router.Route{Path: "/", Component: Index})
    router.RegisterRoute(router.Route{Path: "/about", Component: About})
    router.RegisterRoute(router.Route{Path: "/posts/:id", Component: PostsId})
}

The file is removed again after the build, so it never lands in your
repository.

Naming rules

• index.go maps to the directory root (pages/index.go is /).
  
• [param].go becomes a :param route segment.
  
• Every file must expose a constructor named after the PascalCase form of
  its path, e.g. func About() core.Component for pages/about.go.
  

Wiring it up

Enable the plugin in rfw.json and blank-import the generated package so
the init registrations run:

import _ "your/module/pages"

The plugin rebuilds whenever a file under the pages directory changes. A
custom directory can be configured with {"dir": "routes"} in the plugin
configuration.