Refactor support for entities

Previously, entities were decoded among their surrounding characters
in text nodes.  This produced some caveats, which can be seen in
get-alex/atom-linter-alex#4.

This update adds decoded entities as their own separate text nodes, and
additionally adds support for warning messages relating to errors in
these entities.

For example, `&copy` is valid HTML (and Markdown) but produces a
Parse Error, which in **mdast** results in a message on the VFile.

See wooorm/parse-entities for more information.

Closes GH-66.
Closes get-alex/atom-linter-alex#4

component.json

@@ -23,7 +23,9 @@
     "mathiasbynens/he": "^0.5.0",
     "wooorm/longest-streak": "^1.0.0",
     "wooorm/markdown-table": "^0.4.0",
+    "wooorm/parse-entities": "^1.0.0",
     "jonschlinkert/repeat-string": "^1.5.2",
+    "wooorm/stringify-entities": "^1.0.0",
     "component/trim": "^0.0.1",
     "wooorm/trim-trailing-lines": "^1.0.0",
     "wooorm/unified": "^2.0.0"