Streamline command prompt string generation

[michaelmcallister]

I've added better error handling as well.

I've tried not to change too many things, but I did cleanup some basic things like the HasArgs method

I've moved all the "items" of the command prompt (hostname, username, etc) into one file, and rather than implement a bespoke method like Print I've just implemented a String representation of it, so that calling fmt.Println on the CmdPrompt struct prints the prompt.

Fixes the following bugs
#4 Control+D sends gosh into an infinite loop
#5 Pressing Enter on an empty command will throw a stracktrace