Limit XML formatter targets

[jcubic]

I have an idea for a new feature for jQuery Terminal

XML should work the same as Prism:

    $.terminal.prism_formatters = {
        echo: true,
        animation: true,
        command: true
    };

There should be a way to limit places where formatted is applied.

The task is to add the same mechanism that is in prims.js into xml_formatter.js.