Refactor: Rename 'all_settings' to 'current_settings'

I don't even think this new one is a great name, but I was confused when coming
back to this and seeing `all_settings`, so I thought I'd try a different name.
Refactor: Extract ignored_settings into its own method
Refactor: Rename 'secured' to 'securable'

The items returned by this are not necessarily encrypted.  If `chamber secure`
was never called, that item would still be returned with the unencrypted value
as long as it was secure_able_.

So I decided to go ahead and rename the references to make it a bit more clear.
Feature: Add a filter for translating secure keys

The `DecryptionFilter` is currently responsible for both decrypting values as
well as translating key names from `_secure_whatever` into `whatever`.  This
breaks single-responsibility as well as the fact that we now need that behavior
elsewhere so we're going to break it out.
Refactor: Remove key translation responsibility from DecryptionFilter

Now that we have the `TranslateSecureKeysFilter`, let's use it.
Style: Make whitespace changes
Feature: Add InsecureFilter

This filter is responsible for finding any settings which are supposed to be
secure and filtering only those which have not been encrypted.
Feature: Allow Settings to filter only those which are insecure

This will return only those settings which the user has marked as wanting to
have secured, but is currently not secure.
Bugfix: When running 'chamber secure' only display insecure settings

Previously when you would run `chamber secure`, it would display all settings
which were marked as needing to be secure, whether they were already encrypted
or not.  This caused confusion since variables which were already secured did
not need to be re-encrypted.

This does _not_ solve the problem of the fact that Chamber re-encrypts already
encrypted values.  That fix is coming in a subsequent commit.