Adding support for Bootstrap 4

[bdewong]

This adds pretty basic support for Bootstrap 4. Needed minor changes to the checkable trait to be able to support the way that Boostrap 4 renders the radio and checkbox fields.

Example Radio call:

	{!! Former::radios('type')
		->radios([
			'Option 1' => [
				'name' => 'type',
				'value' => 'o1',
				'class' => 'form-check-input'
			],
			'Option 2' => [
				'name' => 'type',
				'value' => 'o2',
				'class' => 'form-check-input'
			]
		])->inline();
	!!}

[claar]

Wow! Thanks so much @bdewong!

This looks fantastic. Would you be willing to copy TwitterBootstrap3Test.php to TwitterBootstrap4Test.php and update the tests to pass?

[claar]

This file will need renamed from TwitterBootstrap4.php to TwitterBootstrap4Test.php and the tests will need updated to pass.

[bdewong]

Sorry, I created them online to get started. I've gone ahead and updated them and they are now passing.