Support setting ssl_verify_callback

This is to support TLS certificate pinning by providing a custom callback, e.g.:

```ruby
tls_pin = "a-hash-of-the-expected-certificate"

Excon.new(
  url,
  ssl_verify_callback: proc { |preverify_ok, ssl_context|
    Digest::SHA256.new.digest(ssl_context.current_cert.to_der) == tls_pin
  }
)