Feature Request: Support Default Options for beangulp.Injest()

[eliptus]

For import scripts that call beangulp.Injest(), allow them to provide default values for CLI options like downloads and destination.

Example:

    ingest = beangulp.Ingest(importers, hooks, {'downloads': './Downloads'})

[eliptus]

I originally reported this against beancount. This has become more relevant with beangulp since --downloads has been replaced with a positional parameter, thus it is more difficult to detect its absence in the wrapper without baking in knowledge of all archive options.