Refactor to prefer prefixed plug-ins in mdast(1)
Refactor to prefer prefixed plug-ins in mdast(1)

Previously, when a user specified using `foo`, `foo` was preferred
over `mdast-foo`. This change prefers the prefixed `mdast-foo` over
`foo`.

Closes GH-100.

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