Make generic importer more flexible

[dumbPy]

• support multiple postings from source account in generic importers
  closes Does not properly clear transactions with multiple postings on the main account #232
• allow importers to override source description
  closes GenericImporterSource - which data/field is used for the source_desc field? #204
• support non-authoritative generic importer sources
  eg. importer that imports transactions from emails notifications that will later be cleared by monthly account statements.

[Zburatorul]

I'd love to review this but not sure how to test. Could you add a sample configured source please, with some description of the parameters?

[dumbPy]

@Zburatorul Added example showing how to use the changes from this PR.

I have personally been using this for more than a month now, where I make my transactions through an app that sends me email notification for each transaction, that I then import quickly on each run without having to wait for the month end to import transactions, and then these transactions are cleared at the end of the month one I reconcile the monthly statement.

[Zburatorul]

Can you add an empty commit please? I can't figure out how to rerun the build workflow.