[ledger-report] Delay report command expansion

[kevinjfoley]

Delays the expansion of the report command until execution time. This leaves
the command in `ledger-report-cmd' unexpanded, which prevents the expanded
version of the command from being edited / saved by the user.

Resolves #325

[enderw88]

Good idea

[kevinjfoley]

Good idea

Thanks! Anything else needed? I'm not sure what's wrong with the CI, I tried rebasing on the latest changes from master but it looks like it still isn't working.

[bcc32]

I think this PR breaks a couple things. For one thing, it causes the user to be re-prompted for any format specifiers that imply a prompt, such as %(account), every time the report is run (e.g., if they modify and save the journal file). Maybe the most robust way to address that issue would be to cache the expansion of each format specifier in some buffer-local alist or something, or maybe the expanded and unexpanded commands should both be saved so that the appropriate one can be used at each call site.