Minor usability tweaks

[jippi]

First off, thanks for a great library 🎉

I'm currently working on integrating this into my project dottie and this is the accumulated fixes / improvements i've made along the way

• Pass in a base context.Context to (*console.Console).Start() allowing for application/command wide context keys can propagate properly when running commands
• Print errors to ErrOrStderr instead of Stdout
• Correctly surface cancellation cause in console.execute() - it was previously always context.Canceled since you need to call context.Cause() to get the underlying real error
• Use explicit return err in console.execute() to simplify debugging
• Implement custom error types for most errors to ease detection and tracking of them
• Reuse AddHistorySource in AddHistorySourceFile instead of duplicating logic
• Reset flag values (in particular Slice flags) values between runs so they don't accumulate