compileVirtual should not depend on `File` object to template

[unoexperto]

I think TwirlCompiler.compileVirtual should not depend on File object to template because internally it's used only to generate template name. This prevent clients from compiling twirl templates on the fly. In my case I'd like admins of web-service to be able to update templates via API which are stored in DB.

[mkurz]

@unoexperto How about a pull request? 😄