feat: Add UnstructuredOrgModeLoader (#6842)

### Summary

Adds `UnstructuredOrgModeLoader` for processing
[Org-mode](https://en.wikipedia.org/wiki/Org-mode) documents.

### Testing

```python
from langchain.document_loaders import UnstructuredOrgModeLoader

loader = UnstructuredOrgModeLoader(
    file_path="example_data/README.org", mode="elements"
)
docs = loader.load()
print(docs[0])
```

### Reviewers

- @rlancemartin
- @eyurtsev
- @hwchase17

tests/integration_tests/document_loaders/test_org_mode.py

@@ -0,0 +1,15 @@
+import os
+from pathlib import Path
+
+from langchain.document_loaders import UnstructuredOrgModeLoader
+
+EXAMPLE_DIRECTORY = file_path = Path(__file__).parent.parent / "examples"
+
+
+def test_unstructured_org_mode_loader() -> None:
+    """Test unstructured loader."""
+    file_path = os.path.join(EXAMPLE_DIRECTORY, "README.org")
+    loader = UnstructuredOrgModeLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1

tests/integration_tests/examples/README.org

@@ -0,0 +1,27 @@
+* Example Docs
+
+The sample docs directory contains the following files:
+
+-  ~example-10k.html~ - A 10-K SEC filing in HTML format
+-  ~layout-parser-paper.pdf~ - A PDF copy of the layout parser paper
+-  ~factbook.xml~ / ~factbook.xsl~ - Example XML/XLS files that you
+   can use to test stylesheets
+
+These documents can be used to test out the parsers in the library. In
+addition, here are instructions for pulling in some sample docs that are
+too big to store in the repo.
+
+** XBRL 10-K
+
+You can get an example 10-K in inline XBRL format using the following
+~curl~. Note, you need to have the user agent set in the header or the
+SEC site will reject your request.
+
+#+BEGIN_SRC bash
+
+   curl -O \
+     -A '${organization} ${email}'
+     https://www.sec.gov/Archives/edgar/data/311094/000117184321001344/0001171843-21-001344.txt
+#+END_SRC
+
+You can parse this document using the HTML parser.