Global Variables

Some minor changes & additions to the environment must be disclosed:

1. _PVERSION is the global to check your current version of Pluto.
2. _PSOUP is a global boolean you can access to check linkage with Soup.
3. package.path is modified to also search for .pluto files.

if _PVERSION == nil then
    print("Plain Lua detected (".._VERSION..")")
else
    local soup = ""
    if not _PSOUP then
        soup = "not "
    end
    print("Pluto detected (".._PVERSION.."), based on ".._VERSION..", "..soup.."compiled with Soup integration")
end