Add support for UTF-7 decoding

[jcranmer]

Several investigations into real-world charset usage have indicated that UTF-7 is very much a required charset for emails, and TextDecoder is unlikely to natively gain support for UTF-7 since it is a very dangerous charset.

[peteroupc]

Can you look at this?

https://gist.github.com/peteroupc/08c5ecc8131a76062ffe

It's a UTF-7 decoder that takes an array buffer or a string, with test cases.

[lygstate]

Now we use third party decoder, it's possible to add UTF-7 support now.