Gentle reminder about updated swiftly

[cmcgee1024]

Since swiftly is often making round-trips with the swift.org API for various commands, it could also do a brief check to see if it is up-to-date at the same time. If it discovers that it is out-of-date then there can be a warning message prompting someone to run self-update.

[louisunlimited]

Hi! I took a stab at this! See PR for details