A pure Erlang IDNA implementation that folllow the RFC5891.

- support IDNA 2008 and IDNA 2003.
- label validation:
 * check NFC: Label must be in Normalization Form C
 * check hyphen: The Unicode string MUST NOT contain "--" (two consecutive hyphens)
   in the third and fourth character positions and MUST NOT start or end with a "-" (hyphen).
 * Leading Combining Marks: The Unicode string MUST NOT begin with a combining mark
   or combining character (see The Unicode Standard, Section 2.11 Unicode for an exact definition).
 * Contextual Rules: The Unicode string MUST NOT contain any characters whose validity
   is context-dependent, unless the validity is positively confirmed by a contextual rule.
   To check this, each code point identified as CONTEXTJ or CONTEXTO in the Tables document RFC5892
   MUST have a non-null rule. If such a code point is missing a rule, the label is invalid.
   If the rule exists but the result of applying the rule is negative or inconclusive,
   the proposed label is invalid.
 * check BIDI: label contains any characters from scripts that are written
   from right to left, it MUST meet the Bidi criteria rfc5893

• Aleksei Nikiforov

• erlang-idna