Made document metadata more flexible.

* Version bump to 1.12.

* The `Meta` type now takes an association list `[(String, [Block])]`
  instead of three hard-coded parameters for title, authors, date.

* `Text.Pandoc.Definition` exports new `getMeta` and `getMetas` helper
  functions, plus replacements for the old `docTitle`, `docAuthors`,
  and `docDate`.

* `Text.Pandoc.Builder` exports new `setMeta` and `setMetas` helper
  functions, as well as a new `ToBlocks` class.