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-0087.mediawiki

@@ -2,7 +2,7 @@
   BIP: 87
   Layer: Applications
   Title: Hierarchy for Deterministic Multisig Wallets
-  Author: Robert Spigler <RobertSpigler@ProtonMail.ch>
+  Authors: Robert Spigler <RobertSpigler@ProtonMail.ch>
   Status: Complete
   Type: Specification
   Created: 2020-03-11