Support for awaitables

[jtpio]

Ideally we would be able to write code that looks like this:

res = await app.commands.execute('docmanager:new-untitled')
await app.commands.execute('docmanager:open', { 'path': res.path })

So it stays very similar to how we would do things on the TypeScript side.

[jtpio]

Related to https://github.com/jtpio/ipylab/issues/9