New argument (Service) for keychain
New argument (URL)
Refactored Browser#agent to allow arbitrary agent strings
Added Browser#post_with_bearer_token for OAuth requests
Stack#last should return Nothing (not nil) when empty
Additional comments (format syntax) for convert_to_time
Basic Keychain implemented
New action: fetch_with_token
Requiring new arguments and actions
Saper#run_from_file and Recipe#run now accept options for Keychain and Browser instances
Executable now supports `agent` and `keychain` options