Markdown writer does not properly escape list-item-look-alikes

[nichtich]

This is a follow up from more general issue #1685. Given this example.md:

---
test: '1\. inline'
...

1\. inline

Parsed and written back as Markdown (tested with pandoc 1.19.2.1.):

pandoc example.md -t markdown -s

Results in this document:

---
test: '1. inline'
---

1\. inline

Escaping is done properly in the document body but not in the document metadata.

[jgm]

I see why it's doing this, but it's not an easy thing to fix.

The rule that gives us the \. in the body is a special-case rule on Para. If a Para block begins with something that could start an ordered list, we insert the extra backslash.

In the metadata, we don't have a Para; we have a MetaInlines element.

Pandoc (Meta {unMeta = fromList [("test",MetaInlines [Str "1.",Space,Str "inline"])]})

But the Markdown writer can't special-case this, since it doesn't see the MetaInlines; it just gets passed a list of inline elements.

[jgm]

Actually it turned out to have a simple fix, and this also dealt with the linked issue (where you don't explicitly escape the period in the title field).