Build/compile properties to allow callback to extension

I'd like to allow other extensions, in this instance Source Orbit, to manage the build and compile steps.

For example, if the `iproj.json` property was like this:

```json
{
  "compile": "ext:IBM.sourceorbit"
  "build": "ext:IBM.sourceorbit"
}
```

Then I'd love for my extension callbacks to invoked with the correct parameters and then other extensions could deal with the build steps.