weight decay for individual param option added

Useful especially for turning the weight decay of bias off
If you only set individual learning rates, your general learning rate will be 1e-3 by default, and multiplied with your individual learning rates. I pointed out in the code below. 
 ```lua
local lr = config.learningRate or 1e-3
-- (4) learning rate decay (annealing)
local clr = lr / (1 + nevals*lrd)
-- (5) parameter update with single or individual learning rates
if lrs then     -- individual learning rate 
    state.deltaParameters:copy(lrs):cmul(dfdx)
    x:add(-clr, state.deltaParameters)  -- Multiply with the general learining rate. 
else
    x:add(-clr, dfdx)
end
```