Add CLI option to prompt for missing information

A simple implementation would only support string values (i.e. wouldnt support pulling context from JSON or ENV). This can be used to inject sensitive data like passwords.

E.g.

Given ``some-template-file``:

```
password: {{ whatever }}
```

```console
$ bp --prompt some-template-file > some-config
Please specify `whatever':  some-value
$ cat some-config
password: some-value
```