ability to switch between build configurations

I'd like to be able to switch between Debug and Release mode, to, for instance, see errors in branches guarded by `#if DEBUG` guards.

**Describe alternatives you've considered**

I tried setting the configuration in the fsproj file, hoping that the change would be detected and read by fsautocomplete, but it had no effect. I'm not that familiar with the build format, so I may have not configured it correctly, or perhaps fsautocomplete isn't parsing that config.