Returns the contents of the editor in text form.
Gets the text from start to end. If end is 'eof, then the contents are returned from start until the end of the editor.
If flattened? is not #f, then flattened text is returned. See Getting Text for a discussion of flattened vs. non-flattened text.
If force-cr? is not #f and flattened? is not #f, then automatic carriage returns (from word-wrapping) are written into the return string as real carriage returns.