Allow to add custom HTML attributes on buttons

[Florian-B]

This feature allows to add custom attributes on buttons (very useful for tracking or javascript).

{% include '@CraueFormFlow/FormFlow/buttons.html.twig' with {
	craue_formflow_button_attr_finish: {'data-foo': 'bar'},
	craue_formflow_button_attr_reset: {'data-foo': 'bar', 'data-bar': 'foo'},
} %}

Will add data-foo="bar" attribute for finish button and data-foo="bar" data-bar="foo" attributes for reset button

See: #267

[coveralls]

Changes Unknown when pulling c440605 on Florian-B:allow-custom-attr-on-buttons into ** on craue:master**.

[coveralls]

Changes Unknown when pulling c440605 on Florian-B:allow-custom-attr-on-buttons into ** on craue:master**.

[coveralls]

Changes Unknown when pulling 93f2a04 on Florian-B:allow-custom-attr-on-buttons into ** on craue:master**.

[coveralls]

Changes Unknown when pulling 93f2a04 on Florian-B:allow-custom-attr-on-buttons into ** on craue:master**.