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-0155.mediawiki

@@ -2,7 +2,7 @@
   BIP: 155
   Layer: Peer Services
   Title: addrv2 message
-  Author: Wladimir J. van der Laan <laanwj@gmail.com>
+  Authors: Wladimir J. van der Laan <laanwj@gmail.com>
   Status: Deployed
   Type: Specification
   Created: 2019-02-27