Add custom icon to FormButton element

[syberon]

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]

Thank you for this PR