Major Changes

• update CmdStdio APIs, use stdin(), stdout(), stderr() to get Reader/Writer for builtin/custom commands
• process internal: move most of the runtime information from Cmds to Cmd.
• freeze APIs and update docs for 1.0 release
• enable logging support
• log error messages when running command fails
• capture all messages on stderr into logs
• fix builtin command stderr redirection
• report cd command error
• try to capture all stdout/stderr messages if they are redirected to file
• support "|&"
• removed "log" crate dependency for end users
• add checks for multiple redirections for the same input/output [#21]
• spawn a separate thread for builtin/custom pipe-out threads to solve the pipe-full issue [#22]