Indentation characters of the original code are not preserved

docformatter unnecessarily replaces tab characters with spaces. When a file
uses tabs, running docformatter on it changes indentation characters both
inside docstrings and in code outside the docstring.

Reproducer

def some_function(
	a_parameter
):
	"""Important documentation.

	with an indented item
	"""
	...

$ docformatter file.py
--- before/file.py
+++ after/file.py
@@ -1,8 +1,8 @@
 def some_function(
-	a_parameter
+ a_parameter
 ):
 	"""Important documentation.
 
-	with an indented item
-	"""
+ with an indented item
+ """
 	...

Cause

Two independent code paths substitute spaces for the original indentation
characters:

1. _do_add_formatted_docstring and _do_add_unformatted_docstring build
   the indent for the docstring's internal lines as " " * token.start[1],
   based on the docstring's opening line. When the opening line is indented with
   a tab, each tab inside the docstring is replaced by a single space.

2. _do_format_code runs the whole file through tokenize.untokenize,
   which reconstructs inter-token whitespace as " " * col_offset. This means
   that tabs on continuation lines are also replaced by a single space.

Start with _do_add_formatted_docstring and _do_add_unformatted_docstring, then inspect _do_format_code and its use of tokenize.untokenize. Run the provided tab-indented reproducer with docformatter and verify that tabs inside docstrings and on code continuation lines remain tabs while formatting still works.