Add :user_agent hash matcher to the header_matchers plugin
Add delay_build plugin, to delay building the rack app until Roda.app is called

This is only needed in extreme cases, where hundreds of middlware
are added after the route block has been defined.  However, in
those cases, every time a middleware is added, it rebuilds the app,
which is an O(n) operation on the number of middleware, which
means the total time is O(n^2).  By not rebuilding unless build! is
called, this turns into an O(n) operation.
Do not cache templates by default if RACK_ENV is development
Add environments plugin, for handling development/test/production environments
Describe Roda as a toolkit

Depending on the plugins you load when Roda, the resulting code for
the web application can be significantly different.  So Roda is
best described as a toolkit that can be used to build routing tree
web frameworks.

While here, change the RDoc to point to the website instead of GitHub
as the homepage.