lib: --forecast=PERIODICEXPR. Fixes #835, #1236

[adept]

Given that we already have a nice single place where "forecast begin" and "forecast end" is defined, plugging in options that allow overriding one or both of the values is pretty straightforward. I added a test nevertheless

[simonmichael]

Thanks!

[simonmichael]

I'll add more thoughts here, hope it's not too much.

"forecast" or something else ?
As you said last year, "forecast" implies future. Now we're allowing periodic transactions to be generated in the past as well, though that will be less common. We can keep calling it "forecast", I think it's ok. Or we could look for a new name, eg --auto-transactions (and make --auto short for --auto-transactions --auto-postings), or something else; though also with --forecast as an alias.

begin/end or from/to ?
Whatever X we choose above - should the other flags be --X-begin/--X-end, or --X-from/--X-to ? The first is consistent with --begin and --end, helping to show that --begin/--end/--forecast-begin/--forecast-end are the four dates affecting a report. The second reads a little more naturally and grammatically. What's your preference ?

showing future transactions in hledger-ui ?
--forecast has a second function of revealing future-dated transactions in hledger-ui (hidden by default). If we allow any of --forecast, --forecast-begin, --forecast-end to enable periodic transactions, would all of them also do the thing in hledger-ui ? If the "forecast" period is in the past that would be unintuitive.

[adept]

I'd like to keep --forecast because supplying PERIODEXPR does not necessary mean that we are talking about the past -- some accounts could be more in the future than the others, and implicitly taking date of the last transaction in the journal as the start of the forecast might imply that we are "going to the future". In this case, using PERIODEXPR means that we simply restore the status quo.

I've just pushed the rework that makes --forecast accept optional PERIODEXPR. When omitted, it will default to current defaults

[simonmichael]

Thanks!