* Colorize spack info. Adds prominence to preferred version. fixes#2708
This uses 'llnl.util.tty.color' to colorize the output of 'spack info'.
It also displays versions in the order the concretizer would choose
them and shows the preferred in a line on its own and in bold.
* Modified output according to Adam and Denis reviews.
Section titles are not bold + black, but bold + blue. Added a new
section named "Preferred version", which prints the preferred version
in bold characters.
* Further modifications according to Adam and Denis reviews.
After "Homepage:" we now have a single space. Removed newline after each
variant. Preferred version is not in bold fonts anymore. Added a simple
test that just runs the command.
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