Added prereleased luaprompt

Added a luaprompt prerelease version which enables the following
features:

 * Autocompletion adds '.' on non-empty tables
 * Autocompletion adds '(' on functions
 * No autocompletion on hidden elements (elements with '_' prefix)

Special thanks to Dimitris Papavasiliou for being very helpful adding these
features to luaprompt.