This change fixes issue 24

The changes I've made in this pull request introduce a new config setting ``FREEZER_IGNORED_FILES`` to fix issue #24. The ignored files setting takes a list of file and directory names (relative to the destination directory) and expands those into a list of ignored file paths. 

So, for example, if you wanted to ignore all files within the ``.git`` directory of a Git repo and a ``CNAME`` file within the top level of the destination directory, you could add the following to your script:

```python
FREEZER_IGNORED_FILES = ['.git', 'CNAME']
```