Okay so...

Take document 1 + document 2
Do a recursive merge
 - Named properties overwrite scalars
 - This is based on the _target_ type, if the source has a map and the target has a scalar... it overwrites the entire map with the scalar,
   if the target is a map and the source is a scalar, the scalar is tossed away and overwritten

Later, implement ways to control this merge. E.g., "!Replace" to replace an entire block.

Things we might need:
  !Replace -- replace entire block
  !Unset -- remove value, any children, etc



So:
```
AwsWhatever: '2020-01-01'
Resources:
  SecurityGroup:
    VpcId: 1234
    Name: Test
    UseSuperSecurity: True
    Subnets:
      - us-east-1a
      - us-east-1b
    Users:
      Alice: 'aws:arn:1234:alice'
      Bob: 'aws:arn:1234:bob'
```
+
```
Resources:
  SecurityGroup:
    Name: NewGroup
    UseSuperSecurity: !Unset ~
    NewValue: True
    Subnets:
      - us-east-1c
    Users: !Replace
      Charlie: 'aws:arn:1234:charlie'
```
=
```
AwsWhatever: '2020-01-01'
Resources:
  SecurityGroup:
  # Values not specified are unchanged
    VpcId: 1234
  # Scalar values specified are replaced
    Name: NewGroup
  # !Unset removes a value/block/etc
  # UserSuperSecurity:
  # Values that don't exist in the original doc are added
    NewValue: True
  # Arrays are appended
    Subnets:
      - us-east-1a
      - us-east-1b
      - us-east-1c
  # Replace stops the merge and just does a copy from the new document
    Users:
      Charlie: 'aws:arn:1234:charlie'
```