Add a --verbose option for listing labels

[eljobe]

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]

Attempting to merge here https://github.com/drazisil/ghi/pull/4 but would like feedback on the reported issue from CodeClimate

[eljobe]

@drazisil, I updated this pull request to fix the problem you were seeing in CodeClimate

[drazisil]

Merged into my fork.