envconfig.gemspec and Gemfile filled in.
RSpec as development dependency, with .rspec file and empty spec_helper.
Proof-of-concept SMTP support.
Smtp::Custom provider for SMTP_PORT etc in ENV.
Service module extracted from SMTP class.
Provider module, not AbstractProvider class. Consistent with Service.
Eliminated ProviderResolver; see Provider.find()
smtp_spec specifies when nothing relevant in ENV.