feat(snippet): report range of snippets in original text

[laptou]

For my own project, I needed a version of Snippet that provided absolute offsets within the text. This PR does the following:

• allows the user to retrieve the absolute offset
• changes the return value of snippet_from_doc to Option<Snippet> and gets rid of "empty snippets"
• adds new methods snippets_from_doc and snippets which return all of the relevant snippets, not just the best one