[orgmode] Native metadata should be supported

[sverona]

The org compiler presently uses the following workaround to include ReST-style metadata.

#+BEGIN_COMMENT
.. title: My post
.. date: 2021-06-06
#+END_COMMENT

Native org metadata, for instance below, should be supported:

#+title: My post
#+date: 2021-06-06

This allows Emacs to index files in packages such as deft without additional markup.

In principle, this should basically be one regex, but there are a few picky details.

1. How should conflicting metadata (org and ReST in the same file) be handled?
2. Which org export options should be honored within Nikola, and how?
3. How should Nikola frontmatter options that do not correspond to any org option be handled?