add command line args for customing colors

[kyoto7250]

close #9

We can specific colors on the command line by this PR.

If you have more better idea, please let me know :)

# usage
gpustat --custom-colors CName=white --custom-colors CUser=white

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Thank you for your awesome application!

[wookayin]

Thanks, this looks like a good option to add. I have two thoughts:

First, it would be great if one can list multiple color entries in a single flag (with the comma separator), just like fzf supports:

gpustat --custom-colors CName=white
gpustat --custom-colors CName=white,CUser=white,CFoo=bar

Second, the color keys such as CName, CUser were chosen for internal use of string formatting, so I would prefer some universal name that is more consistent with self.entry or json output, e.g. name, memory.used, memory.total. Let me think more what'd be the best.

[kyoto7250]

Thank you for your review.

Let me think more what'd be the best.

Okay, let me know if you come up with a good idea :)