Changed Settings export and boolean handling

* By default, all ROOT settings at or above the chosen export level
  (kEnvLocal by default) are now exported, whether used through
  froast::Settings or not. This also catches settings like ROOT fitter
  defaults, etc., which are usually not accessed directly via froast,
  but can be very important.

* Boolean settings are now set/written as "true" or "false" for increased
  JSON compatibility. Numeric values are still valid in input settings
  (with values >0 interpreted as true).