Validate `--type Boolean`

I ran this command:

```
confcrypt aws add config/Local/Grape.econf --key … --name ROLLBAR_MUTE --value Blah --type Boolean
```

Note: I wrote `--value Blah` and expected it to throw an error.

Start by reproducing the `confcrypt aws add` command from the issue and trace the handling of `--type Boolean`. Done means the invalid value `Blah` is rejected with an error instead of being accepted; the issue names no source file or test, so locating the relevant entry point will require repository exploration.

Written by the indexing model from the issue text.