pandoc.hs: Add --include-early argument option

- Command line option of -E/--include-early and yaml entry of early-includes.
- Include contents as early as validly possible for the target type.
- Along with -H, -B, -A contents can be included before/after template included
  header data and before/after source body data.

Signed-off-by: Thell Fowler <thell@tbfowler.name>