fix Lua console so that it lets you continue incomplete commands

It was always supposed to do this, but at some point (probably the
switch to Lua 5.2) it stopped working. If return is pressed when the
current command is syntactically incomplete, it will insert a newline
and let you continue the command (instead of giving an error message)
Merge pull request #2198 from johnbartholomew/lua-console-incomplete-commands

Fix Lua console so that it lets you continue incomplete commands