Enhancements to WatchedCDXSource

As per discussions in #184, there are some features people would like to see in the `WatchedCDXSource`:
- **Filter** - Probably a regex defining which filenames to consider as CDXs. Default to something like `.*\.cdx$`
- **Recursive** - Boolean, whether or not to traverse sub-directories. This feature makes the filter, potentially, even more useful. Default to `false`.
