Common: Support config files passed on the command line

[istreeter]

Currently the loader and transformer apps only accept base64-encoded configuration passed directly on the command line:

--config=<BASE64 CONFIG HERE> --iglu-config=<BASE64 CONFIG HERE>

However, all other Snowplow applications allow passing configuration from files:

--config=/path/to/config --iglu-config=/path/to/resolver

Some Snowplow apps even support both, so I suggest we do that. The file-based method is more convenient in some deployment setups, e.g. using Kubernetes with ConfigMaps. Note, we recently made the same change in the BigQuery loader so we could copy the same approach.