Support overriding DNS with resolve option

[strzibny]

Support overriding DNS resolution that wreq itself already supports.

Example:

client = Wreq::Client.new(
  resolve: {
    "example.com" => "127.0.0.1",                       # one address
    "api.example.com" => ["10.0.0.1", "10.0.0.2"],      # tried in order
  }
)

resp = client.get("https://example.com/")  # connects to 127.0.0.1:443

This can be helpful when DNS is already resolved as part of a security check.