fix: perform case-insensitive search for identifier

When searching for the comment line after which
the issue URL will be inserted, use a case-insensitive
search for the identifier portion of the line.
This matches how TodoParser identifies TODOs using
a case-insensitive search.

Closes github.com/alstr#224

main.py

@@ -76,7 +76,7 @@ def process_diff(diff, client=Client(), insert_issue_urls=False, parser=TodoPars
                     if line_number < len(file_lines):
                         # Duplicate the line to retain the comment syntax.
                         old_line = file_lines[line_number]
-                        remove = fr'{raw_issue.identifier}.*{raw_issue.title}'
+                        remove = fr'(?i:{raw_issue.identifier}).*{raw_issue.title}'
                         insert = f'Issue URL: {client.get_issue_url(new_issue_number)}'
                         new_line = re.sub(remove, insert, old_line)
                         # make sure the above operation worked as intended