Overiding a format object from a preset should not override the whole object

[Dan503]

For example, the es6 preset format object looks like this:

{
      imports: `import $name from '$path'`,
      functions: '  $name()',
}

If I want to change functions: ' $name()', to something else, I should be able to override that without also overriding the imports: import $name from '$path', value.