process: Author ↦ Authors

```
sed -z -i 's/Author: /Authors: /' bip-0*.md
sed -z -i 's/Author: /Authors: /' bip-0*.mediawiki
```

Also align correctly in case of multiple authors.

bip-0080.mediawiki

@@ -1,8 +1,8 @@
 <pre>
   BIP: 80
   Title: Hierarchy for Non-Colored Voting Pool Deterministic Multisig Wallets
-  Author: Justus Ranvier <justus@opentransactions.org>
-          Jimmy Song <jimmy@monetas.net>
+  Authors: Justus Ranvier <justus@opentransactions.org>
+           Jimmy Song <jimmy@monetas.net>
   Status: Closed
   Type: Informational
   Created: 2014-08-11