Add syntax highlighting to Go example

[ivy]

This pull request adds syntax highlighting to the example in the readme:

package main

import (
        "crypto/tls"

        "github.com/go-mail/gomail"
)

func main() {
        d := gomail.NewDialer("smtp.example.com", 587, "user", "123456")
        d.TLSConfig = &tls.Config{InsecureSkipVerify: true}

        // Send emails using d.
}

From go-gomail#92, part of #1.

[pedromorgan]

@ivy Can you target master instead of v2

[ivy]

@pedromorgan Fixed. You should be good to merge.