Use chrono for time types

[theduke]

Hey, this is currently the best mail parser crate I have found!

It would be great if the create used chrono for date/time types though, since it is pretty much the standard in the ecosystem.

[mikedilger]

Thanks.

I haven't worked on this code in ages, but I believe it uses chrono as an optional dependency, and compiles in the chrono From/Into traits if you enable it.