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Add boolean_label_class config

This config is to make possible to change the class of the label that
wrap the input in a boolean input.

Before this change we always use 'checkbox'. Now the default is still
'checkbox' but it can set to nil
Fix typo
Merge pull request #999 from plataformatec/rm-boolean-label-class

Add boolean_label_class config
Merge branch 'master' into ua-html-options

Conflicts:
	CHANGELOG.md