Support a default Key

While Confcrypt supports multiple keys per-file, often times we only use a single key for the entire file. In that situation, the steps of tracking down a key and including it on the command line are unnecessary friction.

I'd like to extend the existing file format and CLI with the concept of a default key. That should look something like this:
```
confcrypt aws new --name Test --type String --value foobar --use-default-key test.econf
```

Inside the file, you'd find:
```
file_default_key=
... config values below
```
That new key, `file_default_key` should be optional, meaning previous concrypt files should all continue to decrypt.