Add custom icon to FormButton element
Add custom icon to FormButton element

Since in this module removed 'fontAwesome' and 'glyphicon' view helpers
there are problem with icon assigning to buttons appeared.

In this commit i added a support to assign an icon to FormButton element
via direct tag providing in element config.

Supported 2 variants:

1. By providing a scalar value containing tag
'element' => [
    'name' => 'button',
    'type' => Button::class,
    'options' => [
        'label'	=>'button',
	'icon' 	=> '<i class="fas fa-cogs"></i>'
    ]
]

2. By providing an array:
'element' => [
    'name' => 'button',
    'type' => Button::class,
    'options' => [
        'label' => 'button',
	'icon'  => [
	    'icon'     => '<i class="fas-fa-cogs"></i>',
	    'position' => 'prepend'
	]
    ]
]

neilime/zf-twbs-helper-module,neilime/zf-twbs-helper-module