Added `--deep=` switch to dub build

By specifying this flag, you can now build all the dependencies
of a staticLibrary. The default behavior is to only build the
library located in the root directory of the dub configuration
file. This allows better integration with other build systems
which require the libraries to be built upfront.

```
dub build --deep
```

If a staticLibrary A depends on staticLibrary B, and the --deep
flag is specified, dub will output both the A and B libraries.