Build dependencies
When I'm doing a local build, I prefer to keep my build directory free from build artifacts. It makes it easier for me to manage my git repo as I can stage my changes without having to worry about accidentally committing a build artifact. The current behavior of nimble is to create an artifact alongside the source file being compiled.

Would you be amendable to making this configurable somehow? I see a few options:

* As a property in the `.nimble` file for a project. This would mean anyone who builds the project would inherit this configuration, but that isn't necessarily a bad thing
* As a CLI arg. But this would require specifying it any time I did a build, which is inconvenient
* In the `nimble.ini` file. This would make it global, which I'm wary of.
* As an environment variable. Also a global change (or I would have to set it any time I started work)

Also, should the `test` target build to a different location than the `bin` target? And should they be separately configurable?

Cheers