#346 Switch from YAML for TOML

From https://github.com/flapjack/flapjack/pull/692 , but should be against correct branch:

> @ali-graham, @jessereynolds  
> Changes for #346 Switch from YAML for TOML. Specs and Features running green, but had to leave support FLAPJACK_ENV in the test harness and `email.rb`.
>
> NB: This doesn't touch on #285 or #618, but sets the groundwork for implementing these.