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-0116.mediawiki

@@ -2,9 +2,9 @@
   BIP: 116
   Layer: Consensus (soft fork)
   Title: MERKLEBRANCHVERIFY
-  Author: Mark Friedenbach <mark@friedenbach.org>
-          Kalle Alm <kalle.alm@gmail.com>
-          BtcDrak <btcdrak@gmail.com>
+  Authors: Mark Friedenbach <mark@friedenbach.org>
+           Kalle Alm <kalle.alm@gmail.com>
+           BtcDrak <btcdrak@gmail.com>
   Status: Draft
   Type: Specification
   Created: 2017-08-25