allow for a way to skip markdown linting

This is the same parameter as we use in the tor-puppet hooks.

bin/mdl-wrapper

 #!/bin/sh
 
+if [ -n "$SKIP_LINT" ] ; then
+    echo "skipping markdown checks because SKIP_LINT is defined"
+    exit 0
+fi
+
 # This script will pass provided arguments to markdownlint (mdl), with
 # Kramdown warnings enabled, but with the GitLab-specific (and
 # non-standard) [[_TOC_]] blob removed. This only works on single