[NTG-context] Markdown subitems fail

[Marco]

Subitems in markdown behave buggy. Example:

\usemodule [markdown]
\starttext
\startmarkdown
- Some item
- Another item
    - Subitem one
    - Subitem two
\stopmarkdown
\stoptext

Result:

• Some item
• Another item
  startmarkdownitemize[packed]
  startitem Subitem one stopitem stopmarkdownitemize

Using pandoc works fine. I assume the markdown module uses a custom
lua parser internally.

Marco