Zsh: Make color for hostname shell prompt configurable

My own zsh shell prompt theme (using prezto, prompt_wook_setup)
displays the hostname in blue color. But, my workstations share
the setting, so they are not fairly distinguishable.

To improve this, define the shell variable $PROMPT_HOST_COLOR
(blue by default) to specify the color to display the hostname.
It can be set from a local zsh setting file.