Add an example how to use extra-args in the configuration file

[areee]

Could it be possible to add an example to the Configuration file section of README.md about how to use extra-args in peanut.yaml?

For example, I found that this works:

# Configuration for https://pub.dev/packages/peanut

extra-args: "--base-href /base/"

[kevmoo]

PR welcome! 😁