Add the ability to configure settings

[ojarjur]

We should add some sort of support for configuration settings.

Things that I can think of that would make for good options to have settings for:

1. Paths to exclude from snapshotting. For example, you probably don't want to snapshot your ~/.Trash directory
2. A block list of objects to always exclude from bundle exporting/importing
3. A default hash function (when we eventually support more than just sha256)