Allow Faraday client configuration (rebase of #138)

[sabman]

Summary

• Squashed/rebased Allow faraday client configuration #138 onto current master
• Optional Faraday configurer runs for one-shot and keep-alive connections

Original PR: #138 by @srook

Note: 4th positional arg is preserved for fidelity to the original PR; we may want to move this into http_options later.

Test plan

• Specs pass
• Configurer is invoked for default connections
• Keep-alive connections still work