Can retention be exposed as a cmd line parameter?

[jshook]

https://github.com/tureus/graphite-rust/blob/master/src/bin/carbon.rs#L59-L64

[xrl]

@jshook yes, I think that would be an easy change. Put the property in the Args struct here: https://github.com/tureus/graphite-rust/blob/master/src/bin/carbon.rs#L35-L40 and document the argument in the USAGE string (I think the formatting matters): https://github.com/tureus/graphite-rust/blob/master/src/bin/carbon.rs#L19-L32 . Do that and I will be happy to merge!