Adding support for an extra json_encode argument for empty table encoding resolution

[szakharchenko]

The extra argument should be a table which, when looked up with an empty table key, would yield a truthy value if that empty table should be encoded as a JSON empty array ([]) instead of an empty object ({}).

Needless to say, the lookup table's metatable is honored so you can in fact do pretty much any checks in its __index metamethod.

This is more flexible than #15 and doesn't require modifying the encoded data like #25.

[szakharchenko]

Actually, I've found that an even more generic solution is even more useful in presence of table.pack(), etc. Disregard this.