Fix recursive `plugins` key handling in mdast(1)
Fix recursive `plugins` key handling in mdast(1)

Previously, configuration merger would turn all `"plugins"` arrays into
objects, even inside plugin option objects. This patch checks for this
property only at the top scope and not recursively.

Closes GH-99.
Closes GH-101.

tanzania82/remarks,wooorm/remark,eush77/remark,eush77/remark