add support for per-account booking method override

[zacchiro]

Beancount has support for per-account booking methods, with the following syntax:

2016-05-01 open Assets:Vanguard:RGAGX  "AVERAGE"

It would be nice to support for this in ledger2beancount.

Currently, the only workaround possible is quite awkward, as one has to disable automatic declarations for all accounts, and replicate manually all open directives with l2b blocks.

How to support it properly is debatable. My take: require a dedicated account metadata field (overridable with an option or otherwise) and interpret it as the booking method, e.g. (in ledger syntax):

account Assets:Vanguard:RGAGX
    ; booking-method: AVERAGE

[tbm]

OTOH, I don't like introducing meaning that doesn't exist in ledger. OTOH, I like it when ledger and beancount can be used in parallel with l2b and this helps with that (if it's a one time conversion, you could just add the booking method manually).

[tbm]

Sorry for the delay. Do you still need this feature? I think it's ok to add. I can look into it if you still need it.

[zacchiro]

I personally don't need it any more. (But might need it in the future.)