Add ChaCha20 Support

[librehat]

chacha20.js can be helpful.

I may contribute to this project once my laptop arrives

[nekolab]

Thanks~
I'll take a look later and PR is welcomed~

[Jason110go]

Hey，how's it going? Sounds like you really made a great move!

[nekolab]

Sorry, currently I have no time to work on this : (
Since I'm planning to use native client engine replace js engine, so this issue may have lower priority
As always, if you want to give some help, pull request is welcome

[librehat]

I tried to use chacha20.js but couldn't make it work. ;( You might wish to check my WIP commit: https://github.com/librehat/shadowsocks-chromeapp/commit/e53c3015edf933e574d4766cd6c8acd1cfef3924

[nekolab]

@librehat Maybe we need to set a padding to create a block and set a counter like the implementation in C version?

[librehat]

@meowlab I don't think we need to set the counter ourselves (if I understand it correctly from libsodium). As for padding, do we need to create a block? Chacha20 is a stream cipher.

[wongsyrone]

Will https://github.com/jedisct1/libsodium.js help?

[nekolab]

@wongsyrone I don't think so, it seems doesn't contains chacha20 and salsa20 cipher support.