Support a default Key

[ChrisCoffey]

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=<key path/id here>
... config values below

That new key, file_default_key should be optional, meaning previous concrypt files should all continue to decrypt.