Add config file for markdownlint

This config file turns off the "line too long" rule when within a code
block or a table.  Often there is no way to avoid going over 80 lines
in such cases.

.mdl_config.json

@@ -0,0 +1,7 @@
+{
+  "MD013": {
+    "code_blocks": false,
+    "tables": false
+  },
+  "default": true
+}

.pre-commit-config.yaml

@@ -30,6 +30,8 @@ repos:
         # GitHub's autorecognition fu to work, so we should leave it
         # alone.
         exclude: LICENSE.md
+        args:
+          - --config=.mdl_config.json
   - repo: https://github.com/adrienverge/yamllint
     rev: v1.16.0
     hooks: