Expanding charges search route

Hello ! I asked a year ago for a [pull request](https://github.com/vapor-community/stripe-kit/pull/244) merge about charge search route allow to implement expand field. Still no answer, is there something I did wrong ? I'm actually using a fork to get that but this way I don't have your latest release when it get published... Can you consider my PR ? or explain what should I do for it to be considered ? Thank you.