Support overriding diff options in project config (& for individual units)

[encounter]

In objdiff.json, you can now specify global diff options:

"options": {
  "demangler": "gnu_legacy",
  "mips.abi": "n32"
}

See the configuration schema.

Additionally, you can specify these options on a per-unit basis, in case specific architecture options may differ per-unit.