Validate AppleLocale against supported locales

[pi0]

Fixes #28, vercel/hyper#3091, and vercel/hyper#2913 (comment).

MacOS defaults read -g AppleLocale returns invalid POSIX values when the locale is different than language, causing values like en_DE or en_IR. Fortunately, we also have /usr/bin/locale -a which returns a list of supported locales.

This PR fixes issue, by validating locale against supporting locales and falling back to the defaultLocale in case of invalid one.

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Attachment. Perl warning:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LC_ALL = (unset),
        LANG = "en_DE.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

[pi0]

@sindresorhus Tests seem to fail on master too. Opened #34 for it.

[sindresorhus]

Can you fix the merge conflict?

[pi0]

@sindresorhus Thanks for time for review. I made changes and also added a comment.