Allow serialization of only whitelisted keys

Example:

    local a = {
    	foo = 'bar',
    	[1] = 2,
    	[true] = 'only me'
    }

    local options = {
    	keywhitelist = {
    		['foo'] = true,
    		[true] = true
    	},
    	ignore = {
    		['bar'] = true
    	}
    }

    print(serpent.block(a, options))

    -- output:

    {
    	[true] = "only me"
    }