Resolve "Switch to IDNA2008 non-transitional processing (and use libidn2 for that)"

Merged Ondřej Surý requested to merge 26-switch-to-IDNA2008-non-transitional-processing into master

This MR transition from libidn to libidn2 to upgrade IDNA to IDNA2018 (transitional). It contains new dig option +idnin, which allows to process invalid domain names much like dig without IDN support.

Originally submitted as !56 (closed) by @pemensik.

Closes #26 (closed)

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