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-0099.mediawiki

@@ -1,7 +1,7 @@
 <pre>
   BIP: 99
   Title: Motivation and deployment of consensus rule changes ([soft/hard]forks)
-  Author: Jorge Timón <jtimon@jtimon.cc>
+  Authors: Jorge Timón <jtimon@jtimon.cc>
   Status: Closed
   Type: Informational
   Created: 2015-06-20