Updated Commenter feature

**Note for new users coming from Twitter:** I sent a link directly to this pull request because it contains updates to the documentation that have not yet been published to the main documentation site for the repository. The published documentation can be found by following the link at the top of the [main page for this repository](https://github.com/tunnelvisionlabs/vsbase).

* Rename class `Commenter` &rarr; `FormatCommenter` (fixes #18)
* Rename method `GetCommenter` &rarr; `TryCreateCommenter`, and use `ITextBuffer` instead of `ITextView` (fixes #29)
* Support commenting operations in virtual space (fixes #30)
* Updated documentation and examples