Return HtmlString instead of string from BootForm methods

[vedmant]

Return HtmlString instead of string from BootForm methods, the same as LaravelCollective do, it allows to use {{ }} instead of {!! !!}, currently it works with {{ BootForm::open() }} and {{ BootForm::close() }} as it uses LaravelCollective methods, but it fails with {{ BootForm::text() }} and other fields.

[dwightwatson]

Sounds good, happy to accept a PR that makes this change.