Add a --verbose option for listing labels

[drazisil]

When --verbose (-v) is passed to the ghi label -l -v command, it
shows the hex codes for the colors of the labels instead of just using
the color on the background in the terminal.

This allows users to know what colors are already being used in case
they want to make a matching set of labels for their repository.

[drazisil]

@eljobe Can you take a look at https://codeclimate.com/github/drazisil/ghi/pull/4 and see if the reported issue needs to be corrected?

[eljobe]

@drazisil, what it's reporting is that you don't need the |v| at the end of the line. This should be similar to line 26 where there is no block variable for the -d command-line argument.