Add a simple 'Context#GetReferrer', as requested by the People: adapt the goreferrer external package

Former-commit-id: 8f1dce93dd5a2449d806f0b28baf5a8c860193b5

README.md

@@ -209,6 +209,45 @@ func main() {
 id: 1234; page: 1; name: manu; message: this_is_great
 ```
 
+### Extract Referer
+
+```go
+package main
+
+import (
+    "github.com/kataras/iris"
+    "github.com/kataras/iris/context"
+)
+
+func main() {
+    app := iris.New()
+
+    app.Get("/", func(ctx context.Context) /* or iris.Context, it's the same for Go 1.9+. */ {
+
+        // request header "referer" or url parameter "referer".
+        r := ctx.GetReferrer()
+        switch r.Type {
+        case context.ReferrerSearch:
+            ctx.Writef("Search %s: %s\n", r.Label, r.Query)
+            ctx.Writef("Google: %s\n", r.GoogleType)
+        case context.ReferrerSocial:
+            ctx.Writef("Social %s\n", r.Label)
+        case context.ReferrerIndirect:
+            ctx.Writef("Indirect: %s\n", r.URL)
+        }
+    })
+
+    app.Run(iris.Addr(":8080"))
+}
+```
+
+How to `curl`:
+
+```bash
+curl http://localhost:8080?referer=https://twitter.com/Xinterio/status/1023566830974251008
+curl http://localhost:8080?referer=https://www.google.com/search?q=Top+6+golang+web+frameworks&oq=Top+6+golang+web+frameworks
+```
+
 ### Upload files
 
 - [single file upload](_examples/http_request/upload-file/main.go)