Exercise: Improve the color selector

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In the external file where we defined the colors, for each color
keep also a character that will be used to display the menu:

examples/hashes/color_map.txt

yellow y
blue b
black k
green g

When displaying the menu show:

y) yellow
z) brown
b) black
e) blue

and wait till the user selects the appropriate letter.

Allow the user to provide a color on the command line
using the --color option and check if that is one of
the valid colors.

When reading in the file describing the menu options,
check for duplicate use of the same letter.