New option to customize the css inside the addon component

[daguilarm]

If we want to add a custom css to the input-group-addon:

<div class="input-group">
    <input type="text" name="example2">
    <span class="input-group-addon newCss1 newCss2">@domain.com</span>
</div>

Using something like this:

$input->afterAddon('@domain.com')->addAddonCss('newCss1')->addAddonCss('newCss2');

[coveralls]

Coverage increased (+0.2%) to 91.071% when pulling 84064ce on daguilarm:master into f358440 on adamwathan:master.

[coveralls]

Coverage increased (+0.2%) to 91.071% when pulling 252a83e on daguilarm:master into f358440 on adamwathan:master.

[coveralls]

Coverage decreased (-1.6%) to 89.286% when pulling 3eeb7d1 on daguilarm:master into f358440 on adamwathan:master.