Connect to a dev server using ports only (shorthand syntax)

**Describe the solution you'd like**
We would like to support a shorter syntax when connecting to a dev server.

Instead of asking the user to type in:
```
swa start http://localhost:4200 --api http://localhost:7071
```

We can also support:

```
swa start :4200 --api :7071
```

This would assume that these dev servers are running on `http://localhost:PORT`.

Specs:
- This shorthand syntax only works for `http`
- Use ":" before the port number. This would make it easier to parse.

cc @johnpapa @anthonychu