More options for preview_length

[Dalgona]

serum.exs:

%{
  # Take the first n characters.
  preview_length: {:chars, 200}

  # Take the first n words.
  preview_length: {:words, 20}

  # Take the first n paragraphs. (Parses <p> tags)
  preview_length: {:paragraphs, 1}

  # Take the first n characters. (Existing method; left for compatibility)
  preview_length: 200
}