Middleware upgrade (#554)

* Adds root level middleware

* Added todo

* Better way for extensions to be added.

* Bad conflict merge?

api/server/extensions.go

@@ -0,0 +1,30 @@
+package server
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/fnproject/fn/fnext"
+)
+
+// TODO: Move this into `github.com/fnproject/fn` package after main is moved out of root dir.
+var extensions = map[string]fnext.Extension{}
+
+// RegisterExtension registers the extension so it's available, but does not initialize it or anything
+func RegisterExtension(ext fnext.Extension) {
+	extensions[ext.Name()] = ext
+}
+
+// AddExtensionByName This essentially just makes sure the extensions are ordered properly.
+// It could do some initialization if required too.
+func (s *Server) AddExtensionByName(name string) {
+	fmt.Printf("extensions: %+v\n", extensions)
+	e, ok := extensions[name]
+	if !ok {
+		log.Fatalf("Extension %v not registered.\n", name)
+	}
+	err := e.Setup(s)
+	if err != nil {
+		log.Fatalf("Failed to add extension %v: %v\n", name, err)
+	}
+}