API extension points (#473)

* API endpoint extensions working.

extensions example.

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

extensions example.
example main.go.

* Uncommented special handler stuff.

* Added section in docs for extending API linking to example main.go.

* Commented out special_handler test

* Changed to NewFromEnv

examples/extensions/.gitignore

@@ -0,0 +1,2 @@
+/extensions
+/extensions.exe

examples/extensions/README.md

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+# Extensions Example
+
+This example adds extra endpoints to the API. 
+
+## Building and Running
+
+```sh
+go build -o functions
+./functions
+```
+
+Then test with:
+
+```sh
+# First, create an app
+fn apps create myapp
+# And test
+curl http://localhost:8080/v1/custom1
+curl http://localhost:8080/v1/custom2
+curl http://localhost:8080/v1/apps/myapp/custom3
+curl http://localhost:8080/v1/apps/myapp/custom4
+```

examples/extensions/main.go

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+package main
+
+import (
+	"context"
+	"fmt"
+	"html"
+	"net/http"
+
+	"github.com/iron-io/functions/api/models"
+	"github.com/iron-io/functions/api/server"
+)
+
+func main() {
+	ctx := context.Background()
+
+	funcServer := server.NewFromEnv(ctx)
+	// Setup your custom extensions, listeners, etc here
+	funcServer.AddEndpoint("GET", "/custom1", &Custom1Handler{})
+	funcServer.AddEndpointFunc("GET", "/custom2", func(w http.ResponseWriter, r *http.Request) {
+		// fmt.Fprintf(w, "Hello, %q", html.EscapeString(r.URL.Path))
+		fmt.Println("Custom2Handler called")
+		fmt.Fprintf(w, "Hello func, %q", html.EscapeString(r.URL.Path))
+	})
+
+	// the following will be at /v1/apps/:app_name/custom2
+	funcServer.AddAppEndpoint("GET", "/custom3", &Custom3Handler{})
+	funcServer.AddAppEndpointFunc("GET", "/custom4", func(w http.ResponseWriter, r *http.Request, app *models.App) {
+		// fmt.Fprintf(w, "Hello, %q", html.EscapeString(r.URL.Path))
+		fmt.Println("Custom4Handler called")
+		fmt.Fprintf(w, "Hello app %v func, %q", app.Name, html.EscapeString(r.URL.Path))
+	})
+	funcServer.Start(ctx)
+}
+
+type Custom1Handler struct {
+}
+
+func (h *Custom1Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	fmt.Println("Custom1Handler called")
+	fmt.Fprintf(w, "Hello, %q", html.EscapeString(r.URL.Path))
+}
+
+type Custom3Handler struct {
+}
+
+func (h *Custom3Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, app *models.App) {
+	fmt.Println("Custom3Handler called")
+	fmt.Fprintf(w, "Hello app %v, %q", app.Name, html.EscapeString(r.URL.Path))
+}