Adds ‘prefix’ to the beginning of selected lines in ‘text’.
If ‘predicate’ is provided, ‘prefix’ will only be added to the lines where ‘predicate(line)’ is True. If ‘predicate’ is not provided, it will default to adding ‘prefix’ to all non-empty lines that do not consist solely of whitespace characters.
Backported from python 3.3 textwrap library