Rename Brain to Cobe2Brain (with associated API changes)

This is ugly but better reflects the current state of the Go
port.
Add ReplyOptions and Brain.ReplyWithOptions

This allows more advanced direction of the reply process.
Rename "main" to "cobe-cli"

This will make it easier to "go install" the cli by itself.
Add del-stemmer and set-stemmer commands to the CLI

This is for compatibility with the Python cobe