Fixed `lua` command

Fixes issue #11 Passing a single argument to the `lua` command breaks with an error in `ipairs`. This is fixed by cheking if the element passed was a `string` (this is the case when there is a single argument) and putting it in a table before passing it to the `ipairs` function
Fixed lua comand (nil argument)

Fixed `lua` command when no argument is passed to it. It acts as a help, displaying a message with information about the command and a warning on it being dangerous.
Merge pull request #14 from Positive07/patch-3

Fixed `lua` command