Parsing:  Generalized state type on readWith.
Use aeson for json.

Benchmarked:  about twice as slow as json!
Revised Text.Pandoc.Templates to accept JSON contexts.

Currently the library is set up with a shim for association
lists, for compatibility, but this can change when the writers
are changed.

New export: `varListToJSON`.

Removed `Empty`.

Simplified template type to a newtype.
Use new flexible metadata type.

* Depend on pandoc 1.12.
* Added yaml dependency.
* `Text.Pandoc.XML`: Removed `stripTags`.  (API change.)
* `Text.Pandoc.Shared`:  Added `metaToJSON`.
  This will be used in writers to create a JSON object for use
  in the templates from the pandoc metadata.
* Revised readers and writers to use the new Meta type.
* `Text.Pandoc.Options`: Added `Ext_yaml_title_block`.
* Markdown reader:  Added support for YAML metadata block.
  Note that it must come at the beginning of the document.
* `Text.Pandoc.Parsing.ParserState`:  Replace `stateTitle`,
  `stateAuthors`, `stateDate` with `stateMeta`.
* RST reader:  Improved metadata.
  Treat initial field list as metadata when standalone specified.
  Previously ALL fields "title", "author", "date" in field lists
  were treated as metadata, even if not at the beginning.
  Use `subtitle` metadata field for subtitle.
* `Text.Pandoc.Templates`:  Export `renderTemplate'` that takes a string
  instead of a compiled template..
* OPML template:  Use 'for' loop for authors.
* Org template: '#+TITLE:' is inserted before the title.
  Previously the writer did this.