RFC: don't let config files set command to run ?

[Simon Michael]

https://github.com/simonmichael/hledger/pull/2669, just pushed, adds git-style command aliases (custom commands) that can be defined in a config file. I've had this for a day, and I can already say it is a very useful feature!

But there's also a breaking change: to reduce the risk of unexpectedly running shell commands, I have dropped config files' ability to set the command to be run. Now, the command must always be specified on the command line.

I could have added a restriction, but it seems simpler and clearer to drop it. Let me know if you'll be affected, or have other feedback.