Include abstract in Org output.
```
% pandoc --to=org --standalone
---
abstract: |
  This is an *abstract*.

  It has multiple paragraphs.
---
^D
#+begin_abstract
This is an /abstract/.

It has multiple paragraphs.

#+end_abstract
```

Parse abstract environment as *abstract* metadata field.
```
% pandoc --from=org --to=markdown --standalone
#+begin_abstract

This is an /abstract/.

It has multiple paragraphs.
#+end_abstract

Main text
^D
---
abstract: |
  This is an *abstract*.

  It has multiple paragraphs.
---

Main text
```

Uppercase ABSTRACT environment is not moved to metadata.
```
% pandoc --from=org --to=markdown --standalone
#+BEGIN_ABSTRACT
Some text.
#+END_ABSTRACT
^D
::: ABSTRACT
Some text.
:::
```
