Update with new addons features

[daguilarm]

For example:

{!! BootForm::inputGroup('Search date', 'date_start') ->addGroupClass('col-md-2') ->addClass('date') ->afterAddon(icon('date')) ->addAddonClass('input-addon-customize') ->addAddonId('open-modal') ->required() !!}

Now I can open a bootstrap modal, just clicking in the addon, and also, customize the addon with a custom css class.

[coveralls]

Coverage increased (+0.3%) to 91.228% when pulling 6e0e54472a9f6f6e7a552cf8f87052f392d7599c on daguilarm:update-with-addons-features into 6dd67add91909c90c4d02c3f123b8e1ba067206c on adamwathan:master.