Add support for HTML file (#973)

### What problem does this PR solve?

Add support for HTML file

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

deepdoc/parser/__init__.py

@@ -4,3 +4,4 @@ from .pdf_parser import RAGFlowPdfParser as PdfParser, PlainParser
 from .docx_parser import RAGFlowDocxParser as DocxParser
 from .excel_parser import RAGFlowExcelParser as ExcelParser
 from .ppt_parser import RAGFlowPptParser as PptParser
+from .html_parser import RAGFlowHtmlParser as HtmlParser