Configuration

allows configuration through env vars, configure block, or an options hash passed to AppFigures::Client

While I'm advocating for merging options passed in with `default_options`, I wanted to give you a better idiom for using a default option when the key is missing than using `||`

Passing a block to `Hash#fetch` let's you set a default if the key is missing/value is nil:

`@client_key = options.fetch(:client_key) { ENV['client_key'] }`