Ability to set the btn-group class

[Alfredao]

Enhancement of #169

Instead of only setting it to true

'btn-group' => true,

We'll use this array notation, so we can set the class we want.

'btn-group' => [ 'btn-class' => 'btn btn-success', ],

Before it was fixed in the code. All btn-groups had btn-primary class

[coveralls]

Coverage decreased (-0.2%) to 72.201% when pulling 1576cad on Alfredao:master into a39af80 on neilime:master.

[neilime]

Thank you for this new behavior