Nikola Metadata Upgrade

This is a plugin to upgrade metadata in `.meta` files from the old format

(without descriptions) to the new reST-esque format. It only applies to users

with two-file posts (`.meta` files alongside text) in the older format (up to 7

lines of text without any descriptions), used by Nikola before v7.0.0. If you

need to use this plugin, **you will be warned** by `nikola build` — otherwise,

there is no need to install it.