Knob for spaces around subscript colon

*SPACES_AROUND_SUBSCRIPT_COLON*

Use spaces around the subscript / slice operator.  For example:

```python
  my_list[1 : 10 : 2]
```

This Bool option (False by default) inserts spaces around colons when
they're used as a subscript / slice operator.

There's a potential interaction with SPACE_INSIDE_BRACKETS, so there's
an extra test case in TestsForSpacesAroundSubscriptColon to verify the
behaviour with both enabled.
6 files changed