Add missing or pipe | operator to pathRegExp

[arichter83]

I want to use optional pre defined prefixes like these:

var group = FlowRouter.group({
  prefix: "/:partner(one|two|three)?"
})
group.route("/", {name: "base", ...)
group.route("/create", {name: "create", ...)

For this the pipe | is missing in the Router.pathRegExp.