tell user if dat version is out of date

[joehand]

I like how yarn does this:

[juliangruber]

Thinking about implementation, what about we run the version check in a disconnected child process which then writes the most current version to ~/.dat-version.txt. The next time you run a dat command, it will read that and warn you, but you never have to wait for anything.

[mafintosh]

Alternative, print out locally installed dat version on every command.