Typecasting?

I ran into an issue where I was trying to make a kubernetes manifest for a volumeMount and I wanted to make it configurable whether or not the volume was readOnly. I'm just using `--include-env` which means that when I provide the environment variable `false` it gets turned into the string `'false'`.

I ended up doing

```
!Defaults
read_only_bool: !Op [!Var read_only, eq, 'true']
```

But it seems like it would be nice to have like a

```
read_only_bool: !Coerce [Boolean, !Var read_only]
```

That could apply standard yaml type guessing to the string and throw an error if it wasn't right (e.g. if you provided read_only=5 that would be a number and so would throw an error, or if you Coerced to numeric then you would end up with an error if you provided the string 'false'.)