How do I make a custom configuration variable?

I’m sure there’s an obvious answer: I want to create a slightly
customized version of my RoR app. It involves minor tweaks to some
routes, changes to some of the layouts, turning off devise’s
:registerable and other bits scattered around the source code.

I’d like to define a single variable or flag that I can use throughout
the code to create the customized version. Whatever I use, it needs to
be visible to models, controllers, views and tests.

What’s the best practice for doing this? (As an aside, I don’t want
to create a git branch for this – I’m still debugging things and it
would become a PITA to port bugfixes among multiple branches).