Support cancellable (or custom) context.Context

[rinchsan]

• Want to support cancellable (or custom) context.Context for http requests
• Going to be a major release of v2
  • Create /v2 directory and copy existing files to the directory, and then support cancellable context.Context only in /v2 ? (according to the official go documentation)
  • https://blog.golang.org/v2-go-modules
• Pass ctx context.Context as a first argument for query/update/validate methods
  • then pass it to api.do as a first argument