Added a MHTML document loader (#6311)

MHTML is a very interesting format since it's used both for emails but
also for archived webpages. Some scraping projects want to store pages
in disk to process them later, mhtml is perfect for that use case.

This is heavily inspired from the beautifulsoup html loader, but
extracting the html part from the mhtml file.

---------

Co-authored-by: rlm <pexpresss31@gmail.com>

tests/unit_tests/document_loaders/test_mhtml.py

@@ -0,0 +1,25 @@
+from pathlib import Path
+
+import pytest
+
+from langchain.document_loaders.mhtml import MHTMLLoader
+
+HERE = Path(__file__).parent
+EXAMPLES = HERE.parent.parent / "integration_tests" / "examples"
+
+
+@pytest.mark.requires("bs4", "lxml")
+def test_mhtml_loader() -> None:
+    """Test mhtml loader."""
+    file_path = EXAMPLES / "example.mht"
+    loader = MHTMLLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1
+
+    metadata = docs[0].metadata
+    content = docs[0].page_content
+
+    assert metadata["title"] == "LangChain"
+    assert metadata["source"] == str(file_path)
+    assert "LANG CHAIN 🦜️🔗Official Home Page" in content