add `text_without_tags` support to `SubRipFile`

[lsloan]

The SubRipItem class supports two properties for getting the text of subtitles, text and text_without_tags. The latter conveniently returns the text with formatting tags removed.

The SubRipFile class supports the property text for getting the subtitle text, but it does not support one for removing format tags. That is unfortunate.

Please add a text_without_tags property to the SubRipFile class.

Thank you!