Added a liftoffrc config flag to only prompt on attributes with no specified default value

This is especially useful when used with CLI config overrides (#126).

As an example, suppose you have the following `.liftoffrc`:
```ruby
company: ACME Inc.
company_identifier: com.example
author: John Doe
prefix: ACM
strict_prompts: true
```
and you run liftoff, you'l only be prompted for the project name before the project gets created.

I have another incoming PR that combines this with #126 by adding a CLI option `--strict-prompts`, making this infinitely more useful because it allows doing this:
`liftoff -n ProjName --strict-prompts`
and you don't get prompted at all.