Introduce alert shortcodes

[anbuchelva]

Alert messages be written like this

{{< alert info >}} A Sample info message! {{< /alert >}}

{{< alert success >}} A Sample success message! {{< /alert >}}

{{< alert warning >}} A Sample Warning message! {{< /alert >}}

{{< alert danger >}} Its too danger here! {{< /alert >}}

Final output would look like this!

[xiaoheiAh]

LGTM