Was previously attempting to cast directly from []interface{} to
[]string or []int which Go doesn't support. Instead, we iterate over and
cast each value (error'ing if the value is not the correct format).
Fixes#580
Previously, defaults specified as nested keys in a yaml file would not
be recognized, i.e. `top: \n bottom: key` would not be accessible using
the name `top.bottom`, but `top.bottom: key` would. These changes
support using nested keys by traversing the configuration tree if the
key name uses '.' as a delimiter.
