Add Option add-in to customize frames

[klausman]

Can be used like so:

      p := pin.New("Processing...",
          pin.WithSpinnerFrames([]rune{'.', 'o', 'O', '0', 'O', 'o'}),
      )

[yarlson]

Could you please update tests as well?

[klausman]

Could you please update tests as well?

I am not sure how to properly test setting the spinner frames, since they never show up when used non-interactive. And when used interactive, the output is not captured. I could of course make something similar to the Basic Usage test, but with custom frames, to see if the call works at all. The added code doesn't have any error paths, so there's nothing to test there, either.

[yarlson]

Could you please update tests as well?

I am not sure how to properly test setting the spinner frames, since they never show up when used non-interactive. And when used interactive, the output is not captured. I could of course make something similar to the Basic Usage test, but with custom frames, to see if the call works at all. The added code doesn't have any error paths, so there's nothing to test there, either.

Similar to the Basic Usage test would be good for now.

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