attempts to preserve substring indentation

[ntjess]

• fixes Optionally preserve sub-indentation while parsing? #51

The tests pass, but I have no idea if I did it the right way. Hoping this is easy enough for someone more experienced to pick up.

[rr-]

Please create separate tests for strip_initial_whitespace function. Also I don't quite understand its purpose, which tests will help with.

CI is failing on style checks. https://github.com/rr-/docstring_parser/#contributing

[rr-]

Also in Google style if we have:

    """Arguments:
        argument: long description,
            long description 2
    """

I'd expect long description 2 to be indented with 4 spaces (relative to the outer scope that is the argument), and currently it's still indented with 8 (that is relative to the global scope of Arguments:).