travis: add a check for common mistake mediawiki links

Several BIPs have mistaken links in format [text](url) which this travis check will detect and complain about.

scripts/link-format-chk.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2019 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Check wrong mediawiki link format
+
+ECODE=0
+FILES=""
+for fname in $(git diff --name-only HEAD $(git merge-base HEAD master)); do
+    if [[ $fname == *.mediawiki ]]; then
+        GRES=$(grep -n '](' $fname)
+        if [ "$GRES" != "" ]; then
+            if [ $ECODE -eq 0 ]; then
+                >&2 echo "Github Mediawiki format writes link as [URL text], not as [text](url):"
+            fi
+            ECODE=1
+            echo "- $fname:$GRES"
+        fi
+    fi
+done
+exit $ECODE