DUB colored output proposal/showcase

[gdelazzari]

For anyone interested, I'm implementing everything in this branch 
of my fork.

https://github.com/gdelazzari/dub/tree/colored-output

Seems like I managed to cleanly substitute the logging module 
with a brand new one :P actually it's most copy-paste with 
changes to allow colored output, but yeah. I also "imported" 
d-colorize (by copying all its file in Dub's codebase - to avoid 
having a Dub package dependency on Dub itself).

This is the relevant part (the new logging module) if anyone 
wants to contribute with feedback or enhancements, since is some 
of the first D code I'm writing. It's still missing the no-TTY 
detection, but it's coming.

https://github.com/gdelazzari/dub/blob/colored-output/source/dub/logging.d