Expose url args encode/decode functions

[mikz]

Middleware should have access to ngx.decode_args and ngx.encode_args as args.encode and args.decode. Maybe args is not the right name.

/cc @picsoung @kikito

[kikito]

args is a bad name for several things. In singular (arg) it's actually a kind-of-implicit name for functions (like self) in Lua (this functionality seems to be deprecated, but I am not 100% sure).

I would go with uri.encode_query and uri.decode_query. But then the user can not use uri as a variable name (well, he/she can, but then it masks the lib)