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-0079.mediawiki

@@ -2,7 +2,7 @@
   BIP: 79
   Layer: Applications
   Title: Bustapay :: a practical coinjoin protocol
-  Author: Ryan Havar <rhavar@protonmail.com>
+  Authors: Ryan Havar <rhavar@protonmail.com>
   Status: Closed
   Type: Informational
   Created: 2018-10-05