refactor(json): make jsonite optional with build tags (#1026)

* refactor(json): Restore gin support for app engine

Create new folder to support multiple json package.
restore gin support for app engine (disable jsonite through tags)

use jsoniter

$ go build -tags=jsoniter .

use default json

$ go build .

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* rename json file.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* docs(json): add build tags document.

* fix(docs): markdown format.

* fix(json): missing space.

README.md

@@ -88,7 +88,6 @@ BenchmarkZeus_GithubAll 		| 2000 		| 944234 	| 300688 	| 2648
 - [x] Battle tested
 - [x] API frozen, new releases will not break your code.
 
-
 ## Start using it
 
 1. Download and install it:
@@ -141,6 +140,14 @@ $ curl https://raw.githubusercontent.com/gin-gonic/gin/master/examples/basic/mai
 $ go run main.go
 ```
 
+## Build with [jsoniter](https://github.com/json-iterator/go)
+
+Gin use `encoding/json` as default json package but you can change to [jsoniter](https://github.com/json-iterator/go) by build from other tags.
+
+```sh
+$ go build -tags=jsoniter .
+```
+
 ## API Examples
 
 ### Using GET, POST, PUT, PATCH, DELETE and OPTIONS