Extend the statement model to include more statement-related metadata

[andrewshadura]

Statements often include all sorts of metadata, such as the address of the bank, the name and the address of the client, the statement ID/serial number and so on. While I am not sure OFX can represent all that information, it’d be great if the model could handle that, so software using ofstatement as a library, or potentially ISO20022 export code (see #59) could use that info.

[kedder]

We could add more fields as long as they are optional and plugins are not required to fill them in. It is desirable not to break existing plugins in any way. I'm not a expert in financial data structures and I'll need help from one to design a data model that is generic enough for diverse use cases.

Basically, I can imagine ofxstatement as a "generic financial format converter", but I'm not sure I'm able to support such an ambitious goal. After all, the main use case of ofxstatement (for me) is still exporting OFX for GnuCash.