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-0145.mediawiki

@@ -2,7 +2,7 @@
   BIP: 145
   Layer: API/RPC
   Title: getblocktemplate Updates for Segregated Witness
-  Author: Luke Dashjr <luke+bip22@dashjr.org>
+  Authors: Luke Dashjr <luke+bip22@dashjr.org>
   Status: Deployed
   Type: Specification
   Created: 2016-01-30