make --out optional in v2

[Kreijstal]

it's annoying to type it everytime, in v1 we didn't have to do it.

[Storyyeller]

Writing to a directory is annoying because there's no guarantee that the class names will map to valid paths in the host filesystem whereas writing to a single file or a zip will pretty much always work. I assumed that probably noone was using directory output anyway, since it's less reliable and so I could reduce complexity by leaving it out. But it sounds like that's not the case :(