Standardize structure of text to entries to match other entry processors

Add process_single_plaintext_file func etc with similar signatures as
org_to_entries and markdown_to_entries processors

The standardization makes modifications, abstractions easier to create

tests/test_client.py

@@ -1,6 +1,5 @@
 # Standard Modules
 import os
-from io import BytesIO
 from urllib.parse import quote
 
 import pytest