add support for eml file parser (#1768)

### What problem does this PR solve?

add support for eml file parser
#1363

### Type of change

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

---------

Co-authored-by: Zhedong Cen <cenzhedong2@126.com>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

deepdoc/parser/txt_parser.py

@@ -0,0 +1,42 @@
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+from rag.nlp import find_codec,num_tokens_from_string
+
+class RAGFlowTxtParser:
+    def __call__(self, fnm, binary=None, chunk_token_num=128):
+        txt = ""
+        if binary:
+            encoding = find_codec(binary)
+            txt = binary.decode(encoding, errors="ignore")
+        else:
+            with open(fnm, "r") as f:
+                while True:
+                    l = f.readline()
+                    if not l:
+                        break
+                    txt += l
+        return self.parser_txt(txt, chunk_token_num)
+
+    @classmethod
+    def parser_txt(cls, txt, chunk_token_num=128):
+        if type(txt) != str:
+            raise TypeError("txt type should be str!")
+        sections = []
+        for sec in txt.split("\n"):
+            if num_tokens_from_string(sec) > 10 * int(chunk_token_num):
+                sections.append((sec[: int(len(sec) / 2)], ""))
+                sections.append((sec[int(len(sec) / 2) :], ""))
+            else:
+                sections.append((sec, ""))
+        return sections