Need to be able to extend babel config

[valeriakononenko]

I'd like to use decorators. For that I need to add

"plugins": [
      ["@babel/plugin-proposal-decorators", { "legacy": true }]
]

to babel config.
But bob (as far as I read the docs+code and finally understand) can only read babel config or generate its own.

I'd like to use bob-generated babel config + add custom babel plugins to it.