Make metadata out of styled pars

Reopening the pull request for making metadata out of styled pars. The new behavior is as follows:

  - Paragraphs with style `Authors`, `Date`, `Title`, and `Abstract` at the beginning of the document will be processed as document metadata. (This list can be expanded in the future by adding styles and resulting metadata keys to the `metaStyles` map.)

  - The "beginning of the document" here is understood as "until a non-empty paragraph either unstyled, or with a style not included in the above list. So, after a normal body paragraph, an `Authors` paragraph would *not* be added to the metadata. This seems to be in keeping with how most users make use of the above fields, and should prevent surprises.

  - `Authors` has slightly special behavior. Separate paragraphs with the `Authors` style, or one paragraph with a hard linebreak, will both be interpreted as a `metaList`. This makes it difficult to add institutions or other information to the author field (unless you put it on the same line), but any attempts to interpret meaning here will probably cause more trouble than they solve.