Support jsoniter to deal with json

[DarthPestilane]

Wanted to make it like go build -tags="jsoniter" which will replace encoding/json with github.com/json-iterator/go.

And I can send a PR if this feature is acceptable.

[vishr]

@DarthPestilane Before sending a PR can briefly describe potential changes in the code?

[DarthPestilane]

@vishr I will add a new directory(subpackage) named "json" and change "encoding/json" to "github.com/labstack/echo/json" in import ...

[vishr]

I would refrain from adding a new dependency. Having said that, you can easily extend Context to include your custom JSON codec see https://echo.labstack.com/guide/context

[DarthPestilane]

@vishr 👍