muttrc: map Tab to next-unread.

It's more useful to see whatever the next unread message is, whether
it's new or not, than to see the new messages before the other unread
ones.  Map Tab and Esc-Tab to achieve this.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Remove make-blog-article.

It's obsolete, as Asciidoctor supports DocBook 5 now, and blogging is
on a separate platform.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>