Add default value for `depth` parameter

Word on the street is, the optimal value for `depth` is 3 <sup>\[citation needed\]</sup>.

Developers who aren't familiar with how the Tangle works don't really know what value to use for `depth`, so we should probably provide a sensible default.

Make `depth` default to 3 for the following commands:

- `promote_transaction`
- `replay_bundle`
- `send_transfer`
- `send_trytes`