exercism: add shell completion

[parkerbxyz]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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Adds installation of Exercism's shell completion scripts: https://github.com/exercism/cli/tree/master/shell

Note: I was unable to get brew audit --strict <formula> to run locally. The problem seems to be related to a known bug described in Homebrew/brew#5561.