Partial class support in runtime templates

Hello MrJul, 

I hacked together a very raw support for partial classes in runtime templates.

Essentially, I've switched one limitation, no partial class support, for another - members defined in .tt are now reported as duplicates (except for TransformText, which is generated with different name). For me the trade-off is ok, I have lots of runtime templates with partial classes and I tend to put all member declarations to partial classes anyway. Having at least some support is better than none at all.

I don't know if you find it useful - you've most probably thought of the possibility already. Maybe you could align me in better direction of implementing partial class support? I could give it a second try.

Thanks for your great work,

O.