Adding support for parameters in modifiers

(I accidentally closed #68 and didn't find how to re-open it)

Following our discussion in https://github.com/dae/anki/pull/67 , I've implemented the change.. well, arguably it's just a one line change.

It allows templates to be written as such:
{{mod1(param1,param2):mod2(param3):field}}
as well as the original syntax:
{{mod1:mod2:field}}

It means three things:
1-for existing plug-ins relying on 1 modifier without parameter ({{mod:field}}), nothing to change.
2-for existing plug-ins relying on 1 modifier with parameter (which they wrote {{mod:param:field}}), the plug-in can continue as it was, but the templates need to be changed to {{mod(param):field}}
3-for new plug-ins, possibility of use arbitrary number of parameters for arbitrary number of modifiers. NOTE: to ensure backward compatibility with 2, the parameters of {{mod(p1,p2,p3):field}} are passed as "p1,p2,p3" rather than ["p1","p2","p3"]: it is up to the plug-in to then apply extra.split(","), or with any separator of their choice. In other words, there is technically only 1 parameter, what it contains is up to the plug-in. In addition to providing backward compatibility, it also means that plug-ins get more freedom to be creative.