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-0156.mediawiki

@@ -2,12 +2,12 @@
   BIP: 156
   Layer: Peer Services
   Title: Dandelion - Privacy Enhancing Routing
-  Author: Brad Denby <bdenby@cmu.edu>
-          Andrew Miller <soc1024@illinois.edu>
-          Giulia Fanti <gfanti@andrew.cmu.edu>
-          Surya Bakshi <sbakshi3@illinois.edu>
-          Shaileshh Bojja Venkatakrishnan <shaileshh.bv@gmail.com>
-          Pramod Viswanath <pramodv@illinois.edu>
+  Authors: Brad Denby <bdenby@cmu.edu>
+           Andrew Miller <soc1024@illinois.edu>
+           Giulia Fanti <gfanti@andrew.cmu.edu>
+           Surya Bakshi <sbakshi3@illinois.edu>
+           Shaileshh Bojja Venkatakrishnan <shaileshh.bv@gmail.com>
+           Pramod Viswanath <pramodv@illinois.edu>
   Status: Closed
   Type: Specification
   Created: 2017-06-09