Chrome: fix wrong stringification for Reset flag

Fix stringification error when the Reset flag is the only flag given
along with colors.
Powerline-basic: use just Reset chrome now that it is fixed

Stringification for the Reset chrome has been fixed in
91727a3e7f721e3d6cbbb63f39c953aff37efc26. We don't need the ResetFg,
ResetBg, ResetFlag anymore.
Chrome: cache built Chrome::Color objects to reduce memory

Should reduce creation of short lived objects, and speed-up color
creation when a color is rebuilt from different places.