Exercise: Command line parameters

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Take the code from the color selector exercise in the files section and change it so the user can supply the name of the file where the colors are listed using the --file filename option.

If the user supplies an incorrect color name (which is not listed among the accepted colors) give an error message and stop execution.

Allow the user to supply a flag called --force that will override the color-name-validity checking and will allow any color name.