sensible defaults for jco types

[ctaggart]

Creating issue from #530 discussion:

@ctaggart: It would be nice if the wit directory exists, may be just default to use it. So jco types would do the equivelent of what jco types wit -o . does now.

@guybedford: that could be a sensible default actually, basically just if no --wit arg is provided assume wit. Would you consider posting a PR?

[ctaggart]

Where are you usually expecting interfaces to go? Should it be next to src in the root of the project? I tried putting it in src, but does not feel right:

[lachieh]

I've been looking at this in a few places and there doesn't seem to be a generally accepted consensus across the popular packages that include generated types. A common pattern for generated types is a folder called types/<tool>/... or just /types/... and, personally, I think that'd suffice since there's a way to change it.