v0.2.0

Added

• FrontmatterExtractor::extracted() is a new function that returns true once
  the extractor has determined the frontmatter is fully extracted.
• FrontmatterExtractor::extract_buffered() is a new function that advances the
  underlying iterator until the frontmatter has been fully extracted. Unlike
  FrontmatterExtractor::extract(), this function buffers all events so that
  they can be returned via iteration.

These features were inspired by suggestions by @benwis in #3.