errands & run errand (prompts for which errand to run if not specified)

Adds command `bosh errands`:

```
$ bosh errands
+------------------+
| Name             |
+------------------+
| acceptance_tests |
| smoke_tests      |
+------------------+
```

Also, `bosh run errands` can now be run without an explicit errand name; if missing it will prompt the user (or automatically run the only available errand).

E.g. if only one errand in target deployment (as available from the director) it immediately starts it:

```
$ bosh run errand

Director task 21
...
[stdout]
...
[stderr]
...

Errand `tree' completed successfully (exit code 0)
```

use &:can_run_as_errand?