option to use a custom babylon config?

I needed to switch from Typescript to Flow.

For anyone else using flow, I was able to modify the `babylon.config.json` to change `"typescript"` to `"flow"` and then the tool ran perfectly.

In my case it was installed globally by npm at `~/.nvm/versions/node/v12.16.1/lib/node_modules/@ant-design/codemod-v4/bin/babylon.config.json`

Start with the installed bin/babylon.config.json path mentioned in the issue and inspect how the codemod CLI loads that configuration. Define how a caller would select a custom Babylon configuration, including Flow support, and verify that the tool runs with the selected configuration.

Written by the indexing model from the issue text.