Improve lua callbacks call.

Improve lua callbacks call.

Now it is possible to return a value from lua callback and rspamd can
automatically insert the symbol corresponding.

For example:

rspamd_config.TEST = function(task)
...
 return true -- inserts symbol TEST with the default value
...
 return true, 0.5 -- inserts symbol TEST with 0.5 multiplier
...
 return true, "opt1", "opt2" -- insert symbol with the options specified
...
 return true, 0.5, "opt1", "opt2"
...
end