Middleware (#502)

* API endpoint extensions working.

extensions example.

extensions example.

* Added server.NewEnv and some docs for the API extensions example.

extensions example.

extensions example.

* Uncommented special handler stuff.

* First example of middleware.

easier to use.

* Added a special Middleware context to make middleware easier to use.

* Fix tests.

* Cleanup based on PR comments.

examples/middleware/.gitignore

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+/middleware
+/middleware.exe

examples/middleware/README.md

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+# Middleware Example
+
+This example adds a simple authentication middleware to IronFunctions. See [main.go](main.go) for example code. 
+
+## Building and Running
+
+```sh
+go build
+./middleware
+```
+
+Then test with:
+
+```sh
+# First, create an app
+fn apps create myapp
+# And test
+curl http://localhost:8080/v1/apps
+```
+
+You should get a 401 error. 
+
+Add an auth header and it should go through successfully:
+
+```sh
+curl -X GET -H "Authorization: Bearer KlaatuBaradaNikto" http://localhost:8080/v1/apps
+```

examples/middleware/main.go

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+package main
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/iron-io/functions/api/models"
+	"github.com/iron-io/functions/api/server"
+)
+
+func main() {
+	ctx := context.Background()
+
+	funcServer := server.NewEnv(ctx)
+
+	funcServer.AddMiddlewareFunc(func(ctx server.MiddlewareContext, w http.ResponseWriter, r *http.Request, app *models.App) error {
+		start := time.Now()
+		fmt.Println("CustomMiddlewareFunc called at:", start)
+		// TODO: probably need a way to let the chain go forward here and return back to the middleware, for things like timing, etc.
+		ctx.Next()
+		fmt.Println("Duration:", (time.Now().Sub(start)))
+		return nil
+	})
+	funcServer.AddMiddleware(&CustomMiddleware{})
+
+	funcServer.Start(ctx)
+}
+
+type CustomMiddleware struct {
+}
+
+func (h *CustomMiddleware) Serve(ctx server.MiddlewareContext, w http.ResponseWriter, r *http.Request, app *models.App) error {
+	fmt.Println("CustomMiddleware called")
+
+	// check auth header
+	tokenHeader := strings.SplitN(r.Header.Get("Authorization"), " ", 3)
+	if len(tokenHeader) < 2 || tokenHeader[1] != "KlaatuBaradaNikto" {
+		w.WriteHeader(http.StatusUnauthorized)
+		m := map[string]string{"error": "Invalid Authorization token. Sorry!"}
+		json.NewEncoder(w).Encode(m)
+		return errors.New("Invalid authorization token.")
+	}
+	fmt.Println("auth succeeded!")
+	return nil
+}