Make iotile tool more friendly for programmatic control

Right now it's difficult to programmatically determine when the `iotile` tool has finished processing a command since the only clue is that it prompts with a context name for another input.

There should be a global command line flag that allows you to turn off that context specific prompt and replace it with a user specified giant random string as in http multipart form uploads. This string should then be echoed back after the context name to provide a definitive trigger for when iotile has finished work:

```
iotile -d "context_marker"
(root)context_marker
```

In particular:
- [ ] add a `-d` global flag to the iotile tool, that if passed will cause that string to be printed after the context name.
- [ ] When `-d` is specified, no space should be included between `(context)` or after delimiter to allow for easy, unambiguous machine parsing of the context name and the command output of the previous line.