Add interactive mode to hledger

[markokocic]

It would be nice if hledger supported interactive mode, like ledger when started without parameters.

This would allow executing multiple commands interactively in one session, possibly without incurring performance penalty as in #2153

[simonmichael]

I'd be open to adding this mode to the CLI. hledger-ui and hledger-web are another way to avoid re-parsing for each report.

[markokocic]

If journal parsing is the bottleneck, it may make sense to cache the outcome of parsing between invocations.

For example, after the parsing step to save already processed structure on disk alongside the journal file, and next time load it instead of the journal, if journal timestamp didn't change.

[simonmichael]

Ledger did this back in the day, but dropped it again since it wasn't worth the added complications.

I agree it would be interesting to at least try.