`BootstrapRadioCheckboxDecorator` prints input string

[peteristhegreat]

As is, it escapes the input field and writes it as part of the label string.

This snippet fixes it for me:
wtforms_widgets/widgets.py

class BootstrapRadioCheckboxDecorator(WidgetDecorator):
    """
    Renders a field in horizontal layout.

    Horizontal layout is a two column layout, where the label is placed in the
    left column and the field is placed right next to it.
    """
    wrapper_class = None

    def _render(self, field, **kwargs):
        return HTMLString(u''.join([
            u'<div class="', self.wrapper_class, u'">',
            u'<label class="', self.wrapper_class + "-label", u'">',
            self.widget(field, class_=self.wrapper_class + "-input", **kwargs),
            escape(field.label.text),
            u'</label>',
            u'</div>',
        ]))

    def render_basic(self, field, **kwargs):
        return self._render(field, **kwargs)

    def render_horizontal(self, field, **kwargs):
        return HTMLString(u''.join([
            u'<div class="offset-sm-4 col-sm-4">',
            self._render(field, **kwargs),
            u'</div>',
        ]))

    def render_inline(self, field, **kwargs):
        return HTMLString(u''.join([
            u'<div class="', self.wrapper_class + "-inline", u'">',
            u'<label class="', self.wrapper_class + "-label", u'">',
            self.widget(field, class_=self.wrapper_class + "-input", **kwargs),
            escape(field.label.text),
            u'</label>',
            u'</div>',
        ]))

    def __call__(self, field, **kwargs):
        render_mode = kwargs.pop("render_mode", "basic")
        if render_mode == "basic":
            return self.render_basic(field, **kwargs)
        elif render_mode == "horizontal":
            return self.render_horizontal(field, **kwargs)
        elif render_mode == "inline":
            return self.render_inline(field, **kwargs)
        else:
            raise ValueError("Unknown render mode: {0}".format(render_mode))


class BootstrapRadioDecorator(BootstrapRadioCheckboxDecorator):
    wrapper_class = u"form-check"


class BootstrapCheckboxDecorator(BootstrapRadioCheckboxDecorator):
    wrapper_class = u"form-check"

I'll try to make a PR in the near future. Thanks for the awesome library.