Option to include comments in `hledger reg` output

[josephmturner]

Hello! Thank you for this great tool. It makes my life easier regularly.

I'm wondering if it would be a good idea to include an option to hledger reg which would add transaction comments (like described here https://github.com/simonmichael/hledger/issues/828#issuecomment-406685080) as a final column of the output. I don't think this would require any special parsing, like the what was requested in #828, but I understand if this is considered out of scope.

Again, thank you for your maintenance of this project. Super useful.

Joseph