Added a method to pass custom headers and context

[aashishgurung]

1. Objective

Added SetCustomHeaders to pass custom headers to a request.
Added SetContext to pass the context to a request.

Usage

result := &omise.Account{}

// passing custom headers
client.WithCustomHeaders(map[string]string{
      "X-Header": "Header value",
})

// pass the context
client.WithContext(context.Background())

err := client.Do(result, &operations.RetrieveAccount{})

if err != nil {
    // handle error
}