Add CLI option to prompt for missing information

[jcmcken]

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 }}

$ bp --prompt some-template-file > some-config
Please specify `whatever':  some-value
$ cat some-config
password: some-value