Allow customizing explanatory comment

[raxod502]

I'd like to use some different text for the explanatory comment that is posted when an issue is closed. However, currently the only way to do that is to fork the repository and build from source. It would be much more convenient to allow exporting an optional environment variable to override the explanatory comment.

[cvrebert]

Yeah, I'm in favor of it being customizable. Problem is, that implies picking a templating system, and I've no idea which to choose. Recommendations welcomed.

[raxod502]

For something as simple as this, I don't think it would be remiss to just use $DAYS_ELAPSED, $LABEL_NAME, etc. and then allow \$LABEL_NAME for quoting. I don't do Scala but it looks like you can do regex replacement using a function of the match, so this would be pretty trivial implementation-wise.

I'm sure there are more elegant solutions though.

[raxod502]

I wrote a replacement in Python that has this feature: https://github.com/raxod502/tidier.