Special characters in tracker URLs

[angrynode]

The trackers in the magnets/torrents are URLs so they can contain funky characters:

• punycode (IDN): internationalized domain names should probably be punycoded
• IPv6 literals contain brackets surrouding the address (eg. udp://[::1]:3000) which should probably be url-encoded

When doing punycode we might want to apply safety rules such as not mixing UTF8 tables (eg. latin alphabet with greek alphabet) to avoid homoglyph attacks, but that may be too cautious because trackers don't have very important personal information to secure.