Adds --double-indent

PEP 8 states that hanging continuation lines should be indented once, except for blocks (if/def/with/...):

```python
somefunc(
    "foo",
    "bar")

def somefunc(
        param1,
        param2):
    pass

if (someconditionhere and
        someothercondition):
    pass
```

However, it is pretty common (ALTHOUGH NON PEP8 COMPLIANT) to indent twice everywhere instead, eg:

```python
lastfunciswear(
        "foo",
        "bar")
```

Without --double-indent, this would trigger E126. Disabling E126 would also stop pep8 from reporting errors like 9-space indents.