Improve `stripWWW` logic

[gcox]

Fixes #116

• Supports TLDs up to 63 characters. TLDs have the same length restrictions as other DNS labels. There are existing TLDs that are at least 25 characters long.
• Supports single character domains.
• Supports internationalized TLDs (requires allowing for "-" and numeric characters in the TLD): https://en.wikipedia.org/wiki/List_of_Internet_top-level_domains#Internationalized_country_code_top-level_domains

[sindresorhus]

Thanks ;)