Add ability to set template of handler per each type #57

Let's say we want to do something, which is not possible via generic method. The easiest example - return created instances for each found type, and those types have constructors with parameters.
Currently you have to resort to using Activator (which is not compile safe).

public BaseType[] GetTypeInstances(string argument)
{
	return [
		new TypeA(argument),
	    new TypeB(argument)
	];
}

I suggest to parameter HandlerTemplate to ScanForTypesAttribute. Source generator should replace T with full name of found type:

[ScanForTypes(AssignableTo=typeof(BaseType), HanderTemplate = "new T(argument)")]
public partial BaseType[] GetTypeInstances(string argument);

It should support void methods (or methods returning first argument) as well:

[ScanForTypes(AssignableTo=typeof(BaseType), HanderTemplate = "registry.Add(new T(argument))")]
public partial void GetTypeInstances(BaseTypeRegistry registry, string argument);

For this case I suggest to add ';' to the end of the string if it is not there.

It is not supported to have both Handler and HandlerTemplate.