Fix for empty Bash arrays in conjunction with option "nounset"
Use same syntax for features in config and improved info screen

- Variable FEATURES (indexed array) may be used in config files with
  same syntax as in package file or user-override.
- Info now only shows the features which are defined for that package.
  Further more changes from the defaults are visually highlighted.
Added constant PKGBOX_INCLUDE which points to pkgbox includes

- PKGBOX_INCLUDE will be honored when set as environment variable.
- Changed pkgbox_include to use this constant.
Moved package related functionality into file package.sh