Add support for more advanced forms of overriding

[dermotduffy]

• Adds support for config override deletion and setting (in addition to merging)
• Closes #1263
• Breaking change: changes overrides from

overrides:
  - conditions:
      [whatever]
    overrides:
      [config to merge]

overrides:
  - conditions:
      [whatever]
    merge:
      [config to merge]