autoformat markdown with prettier; closes mochajs#3773

Signed-off-by: Christopher Hiller <boneskull@boneskull.com>

.lintstagedrc.json

@@ -1,7 +1,7 @@
 {
   "linters": {
     "(bin/*|*.js|**/*.js)": ["eslint --fix", "git add"],
-    "(*.{json,yml}|**/*.{json,yml})": ["prettier --write", "git add"]
+    "(*.{json,yml,md}|**/*.{json,yml,md})": ["prettier --write", "git add"]
   },
   "ignore": ["docs/**/*.js", "test/**/*.fixture.js", "package*.json"]
 }