aiaiai-email: add [defaults] section to config

aiaiai-email: add [defaults] section to config

This patch adds a [defaults] section for the configuration file which
allows setting shared project settings which might be overridden
per-project. This will be used more in a follow-on patch which adds
additional configuration options.

- v2
* put ini_config_is_set into the libshell file
* use __ instead of _ to indicate internal variable
* update comment to better reflect per-project and default variable
  interaction

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>