Emit a warning when no groups are set

[samatjain]

When no groups are set, dots will happily run w/out error leaving the user to figure out why nothing got installed.

dots should emit a warning when there are no groups set for installation.

[evanpurkhiser]

Warnings shouldn't be printed when the dots script is called from the bash completion script. But that is probably something that could be taken care of by some kind of UI object mentioned in #8, like a --silent flag.

This is "fixed" though.