Fix normalize_paths to allow whitespace

This fixes normalize_paths so that it strips whitespace before and after
paths before working on them. This allows you to do specify excludes
with multiple lines in the config file.

This also tweaks normalize_paths so that the behavior matches the
docstring.

This also adds tests.

Fixes #339
Hi @willkg, I'd be interested to hear what your case was where this was failing for you. I'm tempted to update this to the following to handle the case where it isn't a list or a string coming in. Alternatively, perhaps it would be better to raise a ValueError at that point.

```python
if not value:
    return []
if isinstance(value, list):
    return value
if not isinstance(value, str):
    return []
```