#4 "echo" implemented, "exit" sketched out.

Commands are now passed through the InternalCommandInterceptor, which
reads and executes any internally-supported commands. If a command is an
internal command, then the command is run and a null command is put in
its place so that no other interceptors attempt to operate on the
command.

Currently, the last interceptor of all (which isn't yet an interceptor)
is CmdProcess. In a future version, CmdProcess should be turned into an
Interceptor and included in the interceptor framework. Currently,
CmdProcess is the only interceptor that sends a command prompt, so it
should not be bypassed.