Add ability to filter by file extension

[pgoodrich]

Would like to be able to specify certain file extensions for a Grabbit job (e.g. Grab only the .jpg, .png, .gif files and not .pdf files). Should this be by whitelist or blacklist? Global config level, specifiy individual paths, or global with path override?

[jbornemann]

Similar situation in https://github.com/TWCable/grabbit/issues/111 number 6. May be helpful in the future to be able to specify a query under a path, which would handle this ask, as well as other similar filtering capability.

[jdigger]

I lean toward white-list only, as that's the easiest to understand. Whitelists are explicitly "Use this", where as blacklists are always "Use the whitelist, but here are exceptions."