New plugin: check_drained

[Martin Blais]

Hi!

I hate to break my own rules, but I've added a new plugin (though from an old idea) to v2.

The new plugin "check_drained" will insert zero balance checks for all currencies seen in an account just before a Close directive.

This should in theory be part of the pedantic plugins but I won't change that in v2 (minimize behavior changes).

It will be in the pedantic list in v3.

Background: I finally had a bit of time to update/review my own accounting (thanks to paternity leave) and had a bit of a "wtf!" moment when I realized that this basic check was not included in Beancount. I'd committed the sin of closing non-empty accounts in the past and was closing more. I figured that since this plugin really is core functionality and did not introduce any extra dependencies, I'd include it in the frozen v2.

Cheers,

https://github.com/beancount/beancount/commit/fddbfdad77166c8f8f05b13b08d1d1a5f326e57c