Define encoding of "password" to UTF-8

Problem

The "password" could be passed in various ways, basic configration file, SIP008 online configuration, command line options, and other customized ways. Currently, the encoding of "password" is defined by implementations, for example

• shadowsocks-libev: UTF-8 in basic configuration file, but undefined in command line options
• shadowsocks-rust: UTF-8
• go-shadowsocks2: Undefined, os.Args in Go is a string[], which is an array of byte arrays of input arguments

So it is possible that if user uses libev with command line options and specifies "password" in UTF-16BE, and it shouldn't work with the rust version, because the "password"s are not the same in byte level.

Proposal

1. The "password" is a "string" in UTF-8 encoding
2. Implementations are allowed to accept "password" in other encoding, but they have to convert it to UTF-8 before set to ciphers

Start with the proposal in this issue and review the described password-handling behavior in shadowsocks-libev, shadowsocks-rust, and go-shadowsocks2. Check the existing five-comment discussion before changing the specification; done means the accepted encoding rule and conversion requirement are clearly documented for implementations.