Allow logging of specified environment variables.

[unixmonkey]

This PR allows users to log extra environment information to the oink.log, if environment keys are specified.

If an optional env_vars array is passed to the middleware, the logs will have an additional Environment: line with specified environment keys and values:

Rails.application.middleware.use( Oink::Middleware, env_vars: ['action_dispatch.request_id'] )

Jan 29 18:28:39 vocalhost rails[74025]: Oink Action: home#show
Jan 29 18:28:39 vocalhost rails[74025]: Environment: {"action_dispatch.request_id": "4cc822f8-0d85-4d80-bcae-d94a4567e06c"}
Jan 29 18:28:39 vocalhost rails[74025]: Memory usage: 4825792 | PID: 74025
Jan 29 18:28:39 vocalhost rails[74025]: Instantiation Breakdown: Total: 2 | Page: 1 | User: 1
Jan 29 18:28:39 vocalhost rails[74025]: Oink Log Entry Complete

This is especially useful for logging the request ID, since logs may be distributed among multiple app servers, and timestamps don't always line up exactly.