Forward `authenticate` keyword arguments

[nevans]

• .start and #start now take an auth keyword argument, which is an optional hash of keyword arguments that will be forwarded to #authenticate.
• #authenticate now forwards its keyword arguments (and its block argument) to its chosen authenticator.

Note: this isn't tested in this PR because the tests I originally wrote for it don't work without authenticators that accept keyword arguments. This PR doesn't update existing authenticators or adding any new authenticator. See #71.

[nevans]

@tmtm Thanks for taking the time to review all of my PRs! I've split this PR out from #71, because the other changes in that PR are not as important or uncontroversial as these changes. However, the tests for this code are still in that PR. Please let me know if you'd like me to make any changes (e.g: adding tests directly to this PR or changing anything about the API). Thanks!