Accept a `fetch` option for dependency injection

[swansontec]

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Edge Wallet runs in a WebView, which has CORS limitations. We can get around this by bridging the React Native fetch method into the WebView, but we need some way to inject this method into this library.

[tbfleming]

This library is part of eosjs v16, which is no longer maintained. eosjs v20 supports injecting fetch.

[swansontec]

Oh, interesting. We'll need to look at upgrading, then.