Add missing -composer option.

When composer needs to be set, the plugin task needs to have an option
so the user can specify it.
Add routes.php generation to plugin bake.

Generating a routes.php file for plugins will help guide developers in
the right direction of putting routes in plugins. Also include some
default routes so people get off to a running start.
Add missing template.
Merge pull request #3874 from markstory/router-plugin-bake

3.0 - Router plugin bake