Make the module a table with __call metamethod, add .version field

There is little reason to use `_VERSION` instead of `version`.
`_VERSION` global is used by Lua itself, but module tables don't
have to worry about namespace clashes.

src/argparse.lua

@@ -1389,6 +1389,12 @@ function Parser:pparse(args)
    end
 end
 
-return function(...)
+local argparse = {}
+
+argparse.version = "0.5.0"
+
+setmetatable(argparse, {__call = function(_, ...)
    return Parser(default_cmdline[0]):add_help(true)(...)
-end
+end})
+
+return argparse