Rework adding symbols from lua.

Rework adding symbols from lua.

It is now possible to use something like:

rspamd_config.SYMBOL = function(task) ... end

or even

rspamd_config.SYMBOL = 	{
    			callback = function(task) ... end,
			weight = '1.0', --optional
			priority = '0', --optional
			type = 'callback' --optional
			}