Add experimental Lua support to Ettercap

I'd just like to get this into Ettercap's codebase so @sussurro and I can have some help and input! Lua support is disabled by default. 

- Enabled via ENABLE_LUA=On
- Depends upon LuaJIT
- Will use bundled LuaJIT if system doesn't provide.
- Leveraged via --lua-script command line.
- Currently, Lua scripts can hook to any hook point. Will eventually
  move toward drop-in replacements for C-based protocol dissectors.