Add a case statement in ENV#set for handling Ruby YAML "basic types"

The case statement converts values passed to ENV#set for "basic type" classes to their appropriate String representations. This commit update adds the ability to call ENV['VAR_NAME'] from a Figs application for "basic types" and fixes issue #13.

The "basic types" handled in ENV#set are
- Integer
- Float
- Null
- Boolean
- Time
- Date