[feature request] New merge type that doesn't require a file

[gdankov]

Hey Julz,

The suggestion here is to perform a standard merge, but instead of having to always specify files in the with section, we can directly write the yaml code we would like to merge.

For example, let's say we have the following aviator file:

spruce:
- base: path/to/base.yml
  merge:
  - with_obj:
    foo:
      bar: 42
      baz: true
  to: result.yml

That would merge the base file with the following:

foo:
  bar: 42
  baz: true

Why? Sometimes you want to merge something really tiny (2-3 lines) and creating a separate file for that can be an overkill.

Thanks 🙂